In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains

_{''n''+1} $\backslash geq$ ''a''_{''n''} for all ''n'' ∈ N. If each consecutive term is strictly greater than (>) the previous term then the sequence is called strictly monotonically increasing. A sequence is monotonically decreasing if each consecutive term is less than or equal to the previous one, and is strictly monotonically decreasing if each is strictly less than the previous. If a sequence is either increasing or decreasing it is called a monotone sequence. This is a special case of the more general notion of a

_{n}'') is such that all the terms are less than some real number ''M'', then the sequence is said to be bounded from above. In other words, this means that there exists ''M'' such that for all ''n'', ''a_{n}'' ≤ ''M''. Any such ''M'' is called an ''upper bound''. Likewise, if, for some real ''m'', ''a_{n}'' ≥ ''m'' for all ''n'' greater than some ''N'', then the sequence is bounded from below and any such ''m'' is called a ''lower bound''. If a sequence is both bounded from above and bounded from below, then the sequence is said to be bounded.

_{''nm''} = ''a''_{''n''} ''a''_{''m''} for all pairs ''n'', ''m'' such that ''n'' and ''m'' are _{''n''} = ''na''_{1} for all ''n''. Moreover, a ''multiplicative'' Fibonacci sequence satisfies the recursion relation ''a''_{''n''} = ''a''_{''n''−1} ''a''_{''n''−2}.
* A

If $(c\_n)$ is a sequence such that $a\_n\; \backslash leq\; c\_n\; \backslash leq\; b\_n$ for all $n\; >\; N$

then $(c\_n)$ is convergent, and $\backslash lim\_\; c\_n\; =\; L$. * If a sequence is bounded and

_{1} = 1 and ''x''_{''n''+1} =
is Cauchy, but has no rational limit, cf.

_{i}'' : ''X'' → ''X_{i}'' defined by the equation $p\_i((x\_j)\_)\; =\; x\_i$. Then the product topology on ''X'' is defined to be the _{i}'' are continuous. The product topology is sometimes called the Tychonoff topology.

_{n}'' = 1/

^{''p''} spaces, consisting of the ''p''-power summable sequences, with the ''p''-norm. These are special cases of L^{''p''} spaces for the _{0}, with the sup norm. Any sequence space can also be equipped with the

vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...

. Specifically, the set of ''F''-valued sequences (where ''F'' is a field) is a function space (in fact, a

^{*}, also called ^{+} is the subsemigroup of ''A''^{*} containing all elements except the empty sequence.

vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...

s and

^{∞} of all infinite binary sequences is sometimes called the

The On-Line Encyclopedia of Integer Sequences

(free) {{Authority control Elementary mathematics *

members
Member may refer to:
* Military jury, referred to as "Members" in military jargon
* Element (mathematics), an object that belongs to a mathematical set
* In object-oriented programming, a member of a class
** Field (computer science), entries in ...

(also called ''elements'', or ''terms''). The number of elements (possibly infinite
Infinite may refer to:
Mathematics
* Infinite set, a set that is not a finite set
*Infinity, an abstract concept describing something without any limit
Music
*Infinite (group), a South Korean boy band
*''Infinite'' (EP), debut EP of American m ...

) is called the ''length'' of the sequence. Unlike a set, the same elements can appear multiple times at different positions in a sequence, and unlike a set, the order does matter. Formally, a sequence can be defined as a function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...

from natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''cardinal ...

s (the positions of elements in the sequence) to the elements at each position. The notion of a sequence can be generalized to an indexed family
In mathematics, a family, or indexed family, is informally a collection of objects, each associated with an index from some index set. For example, a ''family of real numbers, indexed by the set of integers'' is a collection of real numbers, wher ...

, defined as a function from an ''arbitrary'' index set.
For example, (M, A, R, Y) is a sequence of letters with the letter 'M' first and 'Y' last. This sequence differs from (A, R, M, Y). Also, the sequence (1, 1, 2, 3, 5, 8), which contains the number 1 at two different positions, is a valid sequence. Sequences can be ''finite
Finite is the opposite of infinite. It may refer to:
* Finite number (disambiguation)
* Finite set, a set whose cardinality (number of elements) is some natural number
* Finite verb, a verb form that has a subject, usually being inflected or marke ...

'', as in these examples, or ''infinite
Infinite may refer to:
Mathematics
* Infinite set, a set that is not a finite set
*Infinity, an abstract concept describing something without any limit
Music
*Infinite (group), a South Korean boy band
*''Infinite'' (EP), debut EP of American m ...

'', such as the sequence of all even positive integers (2, 4, 6, ...).
The position of an element in a sequence is its ''rank'' or ''index''; it is the natural number for which the element is the image. The first element has index 0 or 1, depending on the context or a specific convention. In mathematical analysis
Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, measure (mathematics), measure, infinite sequences, series (m ...

, a sequence is often denoted by letters in the form of $a\_n$, $b\_n$ and $c\_n$, where the subscript ''n'' refers to the ''n''th element of the sequence; for example, the ''n''th element of the Fibonacci sequence
In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

''$F$'' is generally denoted as ''$F\_n$''.
In computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

and computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...

, finite sequences are sometimes called strings, words
A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no conse ...

or lists, the different names commonly corresponding to different ways to represent them in computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...

; infinite sequences are called streams. The empty sequence ( ) is included in most notions of sequence, but may be excluded depending on the context.
Examples and notation

A sequence can be thought of as a list of elements with a particular order. Sequences are useful in a number of mathematical disciplines for studying functions, spaces, and other mathematical structures using theconvergence
Convergence may refer to:
Arts and media Literature
*''Convergence'' (book series), edited by Ruth Nanda Anshen
*Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics:
**A four-part crossover storyline that ...

properties of sequences. In particular, sequences are the basis for series
Series may refer to:
People with the name
* Caroline Series (born 1951), English mathematician, daughter of George Series
* George Series (1920–1995), English physicist
Arts, entertainment, and media
Music
* Series, the ordered sets used in ...

, which are important in differential equations and analysis
Analysis ( : analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (3 ...

. Sequences are also of interest in their own right, and can be studied as patterns or puzzles, such as in the study of prime number
A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

s.
There are a number of ways to denote a sequence, some of which are more useful for specific types of sequences. One way to specify a sequence is to list all its elements. For example, the first four odd numbers form the sequence (1, 3, 5, 7). This notation is used for infinite sequences as well. For instance, the infinite sequence of positive odd integers is written as (1, 3, 5, 7, ...). Because notating sequences with ellipsis
The ellipsis (, also known informally as dot dot dot) is a series of dots that indicates an intentional omission of a word, sentence, or whole section from a text without altering its original meaning. The plural is ellipses. The term origin ...

leads to ambiguity, listing is most useful for customary infinite sequences which can be easily recognized from their first few elements. Other ways of denoting a sequence are discussed after the examples.
Examples

Theprime number
A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

s are the natural numbers greater than 1 that have no divisor
In mathematics, a divisor of an integer n, also called a factor of n, is an integer m that may be multiplied by some integer to produce n. In this case, one also says that n is a multiple of m. An integer n is divisible or evenly divisible by ...

s but 1 and themselves. Taking these in their natural order gives the sequence (2, 3, 5, 7, 11, 13, 17, ...). The prime numbers are widely used in mathematics, particularly in number theory
Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers and integer-valued functions. German mathematician Carl Friedrich Gauss (1777–1855) said, "Mat ...

where many results related to them exist.
The Fibonacci numbers
In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

comprise the integer sequence whose elements are the sum of the previous two elements. The first two elements are either 0 and 1 or 1 and 1 so that the sequence is (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...).
Other examples of sequences include those made up of rational numbers
In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ). The set of all rat ...

, real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...

s and complex numbers
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the form ...

. The sequence (.9, .99, .999, .9999, ...), for instance, approaches the number 1. In fact, every real number can be written as the limit of a sequence of rational numbers (e.g. via its decimal expansion
A decimal representation of a non-negative real number is its expression as a sequence of symbols consisting of decimal digits traditionally written with a single separator:
r = b_k b_\ldots b_0.a_1a_2\ldots
Here is the decimal separator, i ...

). As another example, is the limit of the sequence (3, 3.1, 3.14, 3.141, 3.1415, ...), which is increasing. A related sequence is the sequence of decimal digits of , that is, (3, 1, 4, 1, 5, 9, ...). Unlike the preceding sequence, this sequence does not have any pattern that is easily discernible by inspection.
Another example of sequences is a sequence of functions, where each member of the sequence is a function whose shape is determined by a natural number indexing that function.
The On-Line Encyclopedia of Integer Sequences
The On-Line Encyclopedia of Integer Sequences (OEIS) is an online database of integer sequences. It was created and maintained by Neil Sloane while researching at AT&T Labs. He transferred the intellectual property and hosting of the OEIS to t ...

comprises a large list of examples of integer sequences.
Indexing

Other notations can be useful for sequences whose pattern cannot be easily guessed or for sequences that do not have a pattern such as the digits of . One such notation is to write down a general formula for computing the ''n''th term as a function of ''n'', enclose it in parentheses, and include a subscript indicating the set of values that ''n'' can take. For example, in this notation the sequence of even numbers could be written as $(2n)\_$. The sequence of squares could be written as $(n^2)\_$. The variable ''n'' is called an index, and the set of values that it can take is called theindex set
In mathematics, an index set is a set whose members label (or index) members of another set. For instance, if the elements of a set may be ''indexed'' or ''labeled'' by means of the elements of a set , then is an index set. The indexing consists ...

.
It is often useful to combine this notation with the technique of treating the elements of a sequence as individual variables. This yields expressions like $(a\_n)\_$, which denotes a sequence whose ''n''th element is given by the variable $a\_n$. For example:
:$\backslash begin\; a\_1\; \&=\; 1\backslash text(a\_n)\_\; \backslash \backslash \; a\_2\; \&=\; 2\backslash text\; \backslash \backslash \; a\_3\; \&=\; 3\backslash text\; \backslash \backslash \; \&\backslash ;\backslash ;\backslash vdots\; \backslash \backslash \; a\_\; \&=\; (n-1)\backslash text\; \backslash \backslash \; a\_n\; \&=\; n\backslash text\; \backslash \backslash \; a\_\; \&=\; (n+1)\backslash text\; \backslash \backslash \; \&\backslash ;\backslash ;\; \backslash vdots\; \backslash end$
One can consider multiple sequences at the same time by using different variables; e.g. $(b\_n)\_$ could be a different sequence than $(a\_n)\_$. One can even consider a sequence of sequences: $((a\_)\_)\_$ denotes a sequence whose ''m''th term is the sequence $(a\_)\_$.
An alternative to writing the domain of a sequence in the subscript is to indicate the range of values that the index can take by listing its highest and lowest legal values. For example, the notation $(k^2)\_^$ denotes the ten-term sequence of squares $(1,\; 4,\; 9,\; \backslash ldots,\; 100)$. The limits $\backslash infty$ and $-\backslash infty$ are allowed, but they do not represent valid values for the index, only the supremum or infimum
In mathematics, the infimum (abbreviated inf; plural infima) of a subset S of a partially ordered set P is a greatest element in P that is less than or equal to each element of S, if such an element exists. Consequently, the term ''greatest lo ...

of such values, respectively. For example, the sequence $(a\_n)\_^\backslash infty$ is the same as the sequence $(a\_n)\_$, and does not contain an additional term "at infinity". The sequence $(a\_n)\_^\backslash infty$ is a bi-infinite sequence, and can also be written as $(\backslash ldots,\; a\_,\; a\_0,\; a\_1,\; a\_2,\; \backslash ldots)$.
In cases where the set of indexing numbers is understood, the subscripts and superscripts are often left off. That is, one simply writes $(a\_k)$ for an arbitrary sequence. Often, the index ''k'' is understood to run from 1 to ∞. However, sequences are frequently indexed starting from zero, as in
:$(a\_k)\_^\backslash infty\; =\; (\; a\_0,\; a\_1,\; a\_2,\; \backslash ldots\; ).$
In some cases, the elements of the sequence are related naturally to a sequence of integers whose pattern can be easily inferred. In these cases, the index set may be implied by a listing of the first few abstract elements. For instance, the sequence of squares of odd numbers could be denoted in any of the following ways.
* $(1,\; 9,\; 25,\; \backslash ldots)$
* $(a\_1,\; a\_3,\; a\_5,\; \backslash ldots),\; \backslash qquad\; a\_k\; =\; k^2$
* $(a\_)\_^\backslash infty,\; \backslash qquad\; a\_k\; =\; k^2$
* $(a\_)\_^\backslash infty,\; \backslash qquad\; a\_k\; =\; (2k-1)^2$
* $\backslash left((2k-1)^2\backslash right)\_^\backslash infty$
Moreover, the subscripts and superscripts could have been left off in the third, fourth, and fifth notations, if the indexing set was understood to be the natural numbers. In the second and third bullets, there is a well-defined sequence $(a\_)\_^\backslash infty$, but it is not the same as the sequence denoted by the expression.
Defining a sequence by recursion

Sequences whose elements are related to the previous elements in a straightforward way are often defined usingrecursion
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathemati ...

. This is in contrast to the definition of sequences of elements as functions of their positions.
To define a sequence by recursion, one needs a rule, called ''recurrence relation'' to construct each element in terms of the ones before it. In addition, enough initial elements must be provided so that all subsequent elements of the sequence can be computed by successive applications of the recurrence relation.
The Fibonacci sequence
In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

is a simple classical example, defined by the recurrence relation
:$a\_n\; =\; a\_\; +\; a\_,$
with initial terms $a\_0\; =\; 0$ and $a\_1\; =\; 1$. From this, a simple computation shows that the first ten terms of this sequence are 0, 1, 1, 2, 3, 5, 8, 13, 21, and 34.
A complicated example of a sequence defined by a recurrence relation is Recamán's sequence In mathematics and computer science, Recamán's sequence is a well known sequence defined by a recurrence relation. Because its elements are related to the previous elements in a straightforward way, they are often defined using recursion.
It tak ...

, defined by the recurrence relation
:$\backslash begina\_n\; =\; a\_\; -\; n,\backslash quad\; \backslash text\backslash \backslash a\_n\; =\; a\_\; +\; n,\; \backslash quad\backslash text,\; \backslash end$
with initial term $a\_0\; =\; 0.$
A ''linear recurrence with constant coefficients'' is a recurrence relation of the form
:$a\_n=c\_0\; +c\_1a\_+\backslash dots+c\_k\; a\_,$
where $c\_0,\backslash dots,\; c\_k$ are constants. There is a general method for expressing the general term $a\_n$ of such a sequence as a function of ; see Linear recurrence
In mathematics (including combinatorics, linear algebra, and dynamical systems), a linear recurrence with constant coefficients (also known as a linear recurrence relation or linear difference equation) sets equal to 0 a polynomial that is linear ...

. In the case of the Fibonacci sequence, one has $c\_0=0,\; c\_1=c\_2=1,$ and the resulting function of is given by Binet's formula
In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

.
A holonomic sequence is a sequence defined by a recurrence relation of the form
:$a\_n=c\_1a\_+\backslash dots+c\_k\; a\_,$
where $c\_1,\backslash dots,\; c\_k$ are polynomial
In mathematics, a polynomial is an expression consisting of indeterminates (also called variables) and coefficients, that involves only the operations of addition, subtraction, multiplication, and positive-integer powers of variables. An example ...

s in . For most holonomic sequences, there is no explicit formula for expressing $a\_n$ as a function of . Nevertheless, holonomic sequences play an important role in various areas of mathematics. For example, many special functions
Special functions are particular mathematical functions that have more or less established names and notations due to their importance in mathematical analysis, functional analysis, geometry, physics, or other applications.
The term is defined b ...

have a Taylor series
In mathematics, the Taylor series or Taylor expansion of a function is an infinite sum of terms that are expressed in terms of the function's derivatives at a single point. For most common functions, the function and the sum of its Taylor ser ...

whose sequence of coefficients is holonomic. The use of the recurrence relation allows a fast computation of values of such special functions.
Not all sequences can be specified by a recurrence relation. An example is the sequence of prime number
A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

s in their natural order (2, 3, 5, 7, 11, 13, 17, ...).
Formal definition and basic properties

There are many different notions of sequences in mathematics, some of which (''e.g.'',exact sequence
An exact sequence is a sequence of morphisms between objects (for example, groups, rings, modules, and, more generally, objects of an abelian category) such that the image of one morphism equals the kernel of the next.
Definition
In the context ...

) are not covered by the definitions and notations introduced below.
Definition

In this article, a sequence is formally defined as afunction
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...

whose domain is an interval of integers
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

. This definition covers several different uses of the word "sequence", including one-sided infinite sequences, bi-infinite sequences, and finite sequences (see below for definitions of these kinds of sequences). However, many authors use a narrower definition by requiring the domain of a sequence to be the set of natural numbers. This narrower definition has the disadvantage that it rules out finite sequences and bi-infinite sequences, both of which are usually called sequences in standard mathematical practice. Another disadvantage is that, if one removes the first terms of a sequence, one needs reindexing the remainder terms for fitting this definition. In some contexts, to shorten exposition, the codomain
In mathematics, the codomain or set of destination of a function is the set into which all of the output of the function is constrained to fall. It is the set in the notation . The term range is sometimes ambiguously used to refer to either th ...

of the sequence is fixed by context, for example by requiring it to be the set R of real numbers, the set C of complex numbers, or a topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...

.
Although sequences are a type of function, they are usually distinguished notationally from functions in that the input is written as a subscript rather than in parentheses, that is, rather than . There are terminological differences as well: the value of a sequence at the lowest input (often 1) is called the "first element" of the sequence, the value at the second smallest input (often 2) is called the "second element", etc. Also, while a function abstracted from its input is usually denoted by a single letter, e.g. ''f'', a sequence abstracted from its input is usually written by a notation such as $(a\_n)\_$, or just as $(a\_n).$ Here is the domain, or index set, of the sequence.
Sequences and their limits (see below) are important concepts for studying topological spaces. An important generalization of sequences is the concept of nets. A net is a function from a (possibly uncountable
In mathematics, an uncountable set (or uncountably infinite set) is an infinite set that contains too many elements to be countable. The uncountability of a set is closely related to its cardinal number: a set is uncountable if its cardinal num ...

) directed set
In mathematics, a directed set (or a directed preorder or a filtered set) is a nonempty set A together with a reflexive and transitive binary relation \,\leq\, (that is, a preorder), with the additional property that every pair of elements ha ...

to a topological space. The notational conventions for sequences normally apply to nets as well.
Finite and infinite

The length of a sequence is defined as the number of terms in the sequence. A sequence of a finite length ''n'' is also called an ''n''-tuple. Finite sequences include the empty sequence ( ) that has no elements. Normally, the term ''infinite sequence'' refers to a sequence that is infinite in one direction, and finite in the other—the sequence has a first element, but no final element. Such a sequence is called a singly infinite sequence or a one-sided infinite sequence when disambiguation is necessary. In contrast, a sequence that is infinite in both directions—i.e. that has neither a first nor a final element—is called a bi-infinite sequence, two-way infinite sequence, or doubly infinite sequence. A function from the set Z of ''all''integers
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

into a set, such as for instance the sequence of all even integers ( ..., −4, −2, 0, 2, 4, 6, 8, ... ), is bi-infinite. This sequence could be denoted $(2n)\_^$.
Increasing and decreasing

A sequence is said to be ''monotonically increasing'' if each term is greater than or equal to the one before it. For example, the sequence $(a\_n)\_^$ is monotonically increasing if and only if ''a''monotonic function
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order ...

.
The terms nondecreasing and nonincreasing are often used in place of ''increasing'' and ''decreasing'' in order to avoid any possible confusion with ''strictly increasing'' and ''strictly decreasing'', respectively.
Bounded

If the sequence of real numbers (''aSubsequences

Asubsequence
In mathematics, a subsequence of a given sequence is a sequence that can be derived from the given sequence by deleting some or no elements without changing the order of the remaining elements. For example, the sequence \langle A,B,D \rangle is a ...

of a given sequence is a sequence formed from the given sequence by deleting some of the elements without disturbing the relative positions of the remaining elements. For instance, the sequence of positive even integers (2, 4, 6, ...) is a subsequence of the positive integers (1, 2, 3, ...). The positions of some elements change when other elements are deleted. However, the relative positions are preserved.
Formally, a subsequence of the sequence $(a\_n)\_$ is any sequence of the form $(a\_)\_$, where $(n\_k)\_$ is a strictly increasing sequence of positive integers.
Other types of sequences

Some other types of sequences that are easy to define include: * Aninteger sequence
In mathematics, an integer sequence is a sequence (i.e., an ordered list) of integers.
An integer sequence may be specified ''explicitly'' by giving a formula for its ''n''th term, or ''implicitly'' by giving a relationship between its terms. For ...

is a sequence whose terms are integers.
* A polynomial sequence is a sequence whose terms are polynomials.
* A positive integer sequence is sometimes called multiplicative, if ''a''coprime
In mathematics, two integers and are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1. Consequently, any prime number that divides does not divide , and vice versa. This is equivale ...

. In other instances, sequences are often called ''multiplicative'', if ''a''binary sequence
A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.
A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...

is a sequence whose terms have one of two discrete values, e.g. base 2 values (0,1,1,0, ...), a series of coin tosses (Heads/Tails) H,T,H,H,T, ..., the answers to a set of True or False questions (T, F, T, T, ...), and so on.
Limits and convergence

An important property of a sequence is ''convergence''. If a sequence converges, it converges to a particular value known as the ''limit''. If a sequence converges to some limit, then it is convergent. A sequence that does not converge is divergent. Informally, a sequence has a limit if the elements of the sequence become closer and closer to some value $L$ (called the limit of the sequence), and they become and remain ''arbitrarily'' close to $L$, meaning that given a real number $d$ greater than zero, all but a finite number of the elements of the sequence have a distance from $L$ less than $d$. For example, the sequence $a\_n\; =\; \backslash frac$ shown to the right converges to the value 0. On the other hand, the sequences $b\_n\; =\; n^3$ (which begins 1, 8, 27, …) and $c\_n\; =\; (-1)^n$ (which begins −1, 1, −1, 1, …) are both divergent. If a sequence converges, then the value it converges to is unique. This value is called the limit of the sequence. The limit of a convergent sequence $(a\_n)$ is normally denoted $\backslash lim\_a\_n$. If $(a\_n)$ is a divergent sequence, then the expression $\backslash lim\_a\_n$ is meaningless.Formal definition of convergence

A sequence of real numbers $(a\_n)$ converges to a real number $L$ if, for all $\backslash varepsilon\; >\; 0$, there exists a natural number $N$ such that for all $n\; \backslash geq\; N$ we have :$,\; a\_n\; -\; L,\; <\; \backslash varepsilon.$ If $(a\_n)$ is a sequence of complex numbers rather than a sequence of real numbers, this last formula can still be used to define convergence, with the provision that $,\; \backslash cdot,$ denotes the complex modulus, i.e. $,\; z,\; =\; \backslash sqrt$. If $(a\_n)$ is a sequence of points in ametric space
In mathematics, a metric space is a set together with a notion of '' distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general set ...

, then the formula can be used to define convergence, if the expression $,\; a\_n-L,$ is replaced by the expression $\backslash operatorname(a\_n,\; L)$, which denotes the distance
Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...

between $a\_n$ and $L$.
Applications and important results

If $(a\_n)$ and $(b\_n)$ are convergent sequences, then the following limits exist, and can be computed as follows: * $\backslash lim\_\; (a\_n\; \backslash pm\; b\_n)\; =\; \backslash lim\_\; a\_n\; \backslash pm\; \backslash lim\_\; b\_n$ * $\backslash lim\_\; c\; a\_n\; =\; c\; \backslash lim\_\; a\_n$ for all real numbers $c$ * $\backslash lim\_\; (a\_n\; b\_n)\; =\; \backslash left(\; \backslash lim\_\; a\_n\; \backslash right)\; \backslash left(\; \backslash lim\_\; b\_n\; \backslash right)$ * $\backslash lim\_\; \backslash frac\; =\; \backslash frac$, provided that $\backslash lim\_\; b\_n\; \backslash ne\; 0$ * $\backslash lim\_\; a\_n^p\; =\; \backslash left(\; \backslash lim\_\; a\_n\; \backslash right)^p$ for all $p\; >\; 0$ and $a\_n\; >\; 0$ Moreover: * If $a\_n\; \backslash leq\; b\_n$ for all $n$ greater than some $N$, then $\backslash lim\_\; a\_n\; \backslash leq\; \backslash lim\_\; b\_n$. * (Squeeze Theorem
In calculus, the squeeze theorem (also known as the sandwich theorem, among other names) is a theorem regarding the limit of a function that is trapped between two other functions.
The squeeze theorem is used in calculus and mathematical anal ...

)If $(c\_n)$ is a sequence such that $a\_n\; \backslash leq\; c\_n\; \backslash leq\; b\_n$ for all $n\; >\; N$

then $(c\_n)$ is convergent, and $\backslash lim\_\; c\_n\; =\; L$. * If a sequence is bounded and

monotonic
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of ord ...

then it is convergent.
* A sequence is convergent if and only if all of its subsequences are convergent.
Cauchy sequences

A Cauchy sequence is a sequence whose terms become arbitrarily close together as n gets very large. The notion of a Cauchy sequence is important in the study of sequences inmetric spaces
In mathematics, a metric space is a set together with a notion of ''distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general settin ...

, and, in particular, in real analysis
In mathematics, the branch of real analysis studies the behavior of real numbers, sequences and series of real numbers, and real functions. Some particular properties of real-valued sequences and functions that real analysis studies include conv ...

. One particularly important result in real analysis is ''Cauchy characterization of convergence for sequences'':
:A sequence of real numbers is convergent (in the reals) if and only if it is Cauchy.
In contrast, there are Cauchy sequences of rational numbers
In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ). The set of all rat ...

that are not convergent in the rationals, e.g. the sequence defined by
''x''here
Here is an adverb that means "in, on, or at this place". It may also refer to:
Software
* Here Technologies, a mapping company
* Here WeGo (formerly Here Maps), a mobile app and map website by Here Technologies, Here
Television
* Here TV (form ...

. More generally, any sequence of rational numbers that converges to an irrational number
In mathematics, the irrational numbers (from in- prefix assimilated to ir- (negative prefix, privative) + rational) are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two inte ...

is Cauchy, but not convergent when interpreted as a sequence in the set of rational numbers.
Metric spaces that satisfy the Cauchy characterization of convergence for sequences are called complete metric space
In mathematical analysis, a metric space is called complete (or a Cauchy space) if every Cauchy sequence of points in has a limit that is also in .
Intuitively, a space is complete if there are no "points missing" from it (inside or at the bou ...

s and are particularly nice for analysis.
Infinite limits

In calculus, it is common to define notation for sequences which do not converge in the sense discussed above, but which instead become and remain arbitrarily large, or become and remain arbitrarily negative. If $a\_n$ becomes arbitrarily large as $n\; \backslash to\; \backslash infty$, we write :$\backslash lim\_a\_n\; =\; \backslash infty.$ In this case we say that the sequence diverges, or that it converges to infinity. An example of such a sequence is . If $a\_n$ becomes arbitrarily negative (i.e. negative and large in magnitude) as $n\; \backslash to\; \backslash infty$, we write :$\backslash lim\_a\_n\; =\; -\backslash infty$ and say that the sequence diverges or converges to negative infinity.Series

A series is, informally speaking, the sum of the terms of a sequence. That is, it is an expression of the form $\backslash sum\_^\backslash infty\; a\_n$ or $a\_1\; +\; a\_2\; +\; \backslash cdots$, where $(a\_n)$ is a sequence of real or complex numbers. The partial sums of a series are the expressions resulting from replacing the infinity symbol with a finite number, i.e. the ''N''th partial sum of the series $\backslash sum\_^\backslash infty\; a\_n$ is the number :$S\_N\; =\; \backslash sum\_^N\; a\_n\; =\; a\_1\; +\; a\_2\; +\; \backslash cdots\; +\; a\_N.$ The partial sums themselves form a sequence $(S\_N)\_$, which is called the sequence of partial sums of the series $\backslash sum\_^\backslash infty\; a\_n$. If the sequence of partial sums converges, then we say that the series $\backslash sum\_^\backslash infty\; a\_n$ is convergent, and the limit $\backslash lim\_\; S\_N$ is called the value of the series. The same notation is used to denote a series and its value, i.e. we write $\backslash sum\_^\backslash infty\; a\_n\; =\; \backslash lim\_\; S\_N$.Use in other fields of mathematics

Topology

Sequences play an important role in topology, especially in the study ofmetric spaces
In mathematics, a metric space is a set together with a notion of ''distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general settin ...

. For instance:
* A metric space
In mathematics, a metric space is a set together with a notion of '' distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general set ...

is compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in British ...

exactly when it is sequentially compact
In mathematics, a topological space ''X'' is sequentially compact if every sequence of points in ''X'' has a convergent subsequence converging to a point in X.
Every metric space is naturally a topological space, and for metric spaces, the notio ...

.
* A function from a metric space to another metric space is continuous exactly when it takes convergent sequences to convergent sequences.
* A metric space is a connected space if and only if, whenever the space is partitioned into two sets, one of the two sets contains a sequence converging to a point in the other set.
* A topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...

is separable exactly when there is a dense sequence of points.
Sequences can be generalized to nets or filters
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...

. These generalizations allow one to extend some of the above theorems to spaces without metrics.
Product topology

The topological product of a sequence of topological spaces is the cartesian product of those spaces, equipped with anatural topology
In any domain of mathematics, a space has a natural topology if there is a topology on the space which is "best adapted" to its study within the domain in question. In many cases this imprecise definition means little more than the assertion that ...

called the product topology
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-s ...

.
More formally, given a sequence of spaces $(X\_i)\_$, the product space
:$X\; :=\; \backslash prod\_\; X\_i,$
is defined as the set of all sequences $(x\_i)\_$ such that for each ''i'', $x\_i$ is an element of $X\_i$. The canonical projections are the maps ''pcoarsest topology
In topology and related areas of mathematics, the set of all possible topologies on a given set forms a partially ordered set. This order relation can be used for comparison of the topologies.
Definition
A topology on a set may be defined as t ...

(i.e. the topology with the fewest open sets) for which all the projections ''pAnalysis

Inanalysis
Analysis ( : analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (3 ...

, when talking about sequences, one will generally consider sequences of the form
:$(x\_1,\; x\_2,\; x\_3,\; \backslash dots)\backslash text(x\_0,\; x\_1,\; x\_2,\; \backslash dots)$
which is to say, infinite sequences of elements indexed by natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called ''cardinal ...

s.
A sequence may start with an index different from 1 or 0. For example, the sequence defined by ''xlog
Log most often refers to:
* Trunk (botany), the stem and main wooden axis of a tree, called logs when cut
** Logging, cutting down trees for logs
** Firewood, logs used for fuel
** Lumber or timber, converted from wood logs
* Logarithm, in mathe ...

(''n'') would be defined only for ''n'' ≥ 2. When talking about such infinite sequences, it is usually sufficient (and does not change much for most considerations) to assume that the members of the sequence are defined at least for all indices large enough
In the mathematical areas of number theory and analysis, an infinite sequence or a function is said to eventually have a certain property, if it doesn't have the said property across all its ordered instances, but will after some instances have pas ...

, that is, greater than some given ''N''.
The most elementary type of sequences are numerical ones, that is, sequences of real
Real may refer to:
Currencies
* Brazilian real (R$)
* Central American Republic real
* Mexican real
* Portuguese real
* Spanish real
* Spanish colonial real
Music Albums
* ''Real'' (L'Arc-en-Ciel album) (2000)
* ''Real'' (Bright album) (2010) ...

or complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...

numbers. This type can be generalized to sequences of elements of some vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...

. In analysis, the vector spaces considered are often function spaces. Even more generally, one can study sequences with elements in some topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...

.
Sequence spaces

Asequence space
In functional analysis and related areas of mathematics, a sequence space is a vector space whose elements are infinite sequences of real or complex numbers. Equivalently, it is a function space whose elements are functions from the natural nu ...

is a vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...

whose elements are infinite sequences of real
Real may refer to:
Currencies
* Brazilian real (R$)
* Central American Republic real
* Mexican real
* Portuguese real
* Spanish real
* Spanish colonial real
Music Albums
* ''Real'' (L'Arc-en-Ciel album) (2000)
* ''Real'' (Bright album) (2010) ...

or complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...

numbers. Equivalently, it is a function space whose elements are functions from the natural numbers to the field
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...

''K'', where ''K'' is either the field of real numbers or the field of complex numbers. The set of all such functions is naturally identified with the set of all possible infinite sequences with elements in ''K'', and can be turned into a vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...

under the operations of pointwise addition In mathematics, the qualifier pointwise is used to indicate that a certain property is defined by considering each value f(x) of some function f. An important class of pointwise concepts are the ''pointwise operations'', that is, operations defined ...

of functions and pointwise scalar multiplication. All sequence spaces are linear subspaces of this space. Sequence spaces are typically equipped with a norm
Naturally occurring radioactive materials (NORM) and technologically enhanced naturally occurring radioactive materials (TENORM) consist of materials, usually industrial wastes or by-products enriched with radioactive elements found in the envi ...

, or at least the structure of a topological vector space
In mathematics, a topological vector space (also called a linear topological space and commonly abbreviated TVS or t.v.s.) is one of the basic structures investigated in functional analysis.
A topological vector space is a vector space that is als ...

.
The most important sequences spaces in analysis are the ℓcounting measure In mathematics, specifically measure theory, the counting measure is an intuitive way to put a measure on any set – the "size" of a subset is taken to be the number of elements in the subset if the subset has finitely many elements, and infinity ...

on the set of natural numbers. Other important classes of sequences like convergent sequences or null sequence
As the positive integer n becomes larger and larger, the value n\cdot \sin\left(\tfrac1\right) becomes arbitrarily close to 1. We say that "the limit of the sequence n\cdot \sin\left(\tfrac1\right) equals 1."
In mathematics, the limit ...

s form sequence spaces, respectively denoted ''c'' and ''c''topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...

of pointwise convergence
In mathematics, pointwise convergence is one of various senses in which a sequence of functions can converge to a particular function. It is weaker than uniform convergence, to which it is often compared.
Definition
Suppose that X is a set and ...

, under which it becomes a special kind of Fréchet space
In functional analysis and related areas of mathematics, Fréchet spaces, named after Maurice Fréchet, are special topological vector spaces.
They are generalizations of Banach spaces ( normed vector spaces that are complete with respect to th ...

called an FK-space
In functional analysis and related areas of mathematics a FK-space or Fréchet coordinate space is a sequence space equipped with a topological structure such that it becomes a Fréchet space. FK-spaces with a normable topology are called BK-spa ...

.
Linear algebra

Sequences over afield
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...

may also be viewed as vectors in a product space
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-seemi ...

) of ''F''-valued functions over the set of natural numbers.
Abstract algebra

Abstract algebra employs several types of sequences, including sequences of mathematical objects such as groups or rings.Free monoid

If ''A'' is a set, thefree monoid In abstract algebra, the free monoid on a set is the monoid whose elements are all the finite sequences (or strings) of zero or more elements from that set, with string concatenation as the monoid operation and with the unique sequence of zero ele ...

over ''A'' (denoted ''A''Kleene star
In mathematical logic and computer science, the Kleene star (or Kleene operator or Kleene closure) is a unary operation, either on sets of strings or on sets of symbols or characters. In mathematics,
it is more commonly known as the free monoid ...

of ''A'') is a monoid
In abstract algebra, a branch of mathematics, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being 0.
Monoid ...

containing all the finite sequences (or strings) of zero or more elements of ''A'', with the binary operation of concatenation. The free semigroup In abstract algebra, the free monoid on a set is the monoid whose elements are all the finite sequences (or strings) of zero or more elements from that set, with string concatenation as the monoid operation and with the unique sequence of zero ele ...

''A''Exact sequences

In the context ofgroup theory
In abstract algebra, group theory studies the algebraic structures known as group (mathematics), groups.
The concept of a group is central to abstract algebra: other well-known algebraic structures, such as ring (mathematics), rings, field ...

, a sequence
:$G\_0\; \backslash ;\backslash xrightarrow\backslash ;\; G\_1\; \backslash ;\backslash xrightarrow\backslash ;\; G\_2\; \backslash ;\backslash xrightarrow\backslash ;\; \backslash cdots\; \backslash ;\backslash xrightarrow\backslash ;\; G\_n$
of groups
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...

and group homomorphism
In mathematics, given two groups, (''G'', ∗) and (''H'', ·), a group homomorphism from (''G'', ∗) to (''H'', ·) is a function ''h'' : ''G'' → ''H'' such that for all ''u'' and ''v'' in ''G'' it holds that
: h(u*v) = h(u) \cdot h(v)
w ...

s is called exact, if the image (or range
Range may refer to:
Geography
* Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra)
** Mountain range, a group of mountains bordered by lowlands
* Range, a term used to i ...

) of each homomorphism is equal to the kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine learn ...

of the next:
:$\backslash mathrm(f\_k)\; =\; \backslash mathrm(f\_)$
The sequence of groups and homomorphisms may be either finite or infinite.
A similar definition can be made for certain other algebraic structures. For example, one could have an exact sequence of linear map
In mathematics, and more specifically in linear algebra, a linear map (also called a linear mapping, linear transformation, vector space homomorphism, or in some contexts linear function) is a mapping V \to W between two vector spaces that pr ...

s, or of modules
Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...

and module homomorphism In algebra, a module homomorphism is a function between modules that preserves the module structures. Explicitly, if ''M'' and ''N'' are left modules over a ring ''R'', then a function f: M \to N is called an ''R''-''module homomorphism'' or an '' ...

s.
Spectral sequences

In homological algebra andalgebraic topology
Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...

, a spectral sequence is a means of computing homology groups by taking successive approximations. Spectral sequences are a generalization of exact sequence
An exact sequence is a sequence of morphisms between objects (for example, groups, rings, modules, and, more generally, objects of an abelian category) such that the image of one morphism equals the kernel of the next.
Definition
In the context ...

s, and since their introduction by , they have become an important research tool, particularly in homotopy theory.
Set theory

An ordinal-indexed sequence is a generalization of a sequence. If α is alimit ordinal
In set theory, a limit ordinal is an ordinal number that is neither zero nor a successor ordinal. Alternatively, an ordinal λ is a limit ordinal if there is an ordinal less than λ, and whenever β is an ordinal less than λ, then there exists a ...

and ''X'' is a set, an α-indexed sequence of elements of ''X'' is a function from α to ''X''. In this terminology an ω-indexed sequence is an ordinary sequence.
Computing

Incomputer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...

, finite sequences are called lists. Potentially infinite sequences are called streams. Finite sequences of characters or digits are called strings.
Streams

Infinite sequences of digits (orcharacters
Character or Characters may refer to:
Arts, entertainment, and media Literature
* ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk
* ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...

) drawn from a finite
Finite is the opposite of infinite. It may refer to:
* Finite number (disambiguation)
* Finite set, a set whose cardinality (number of elements) is some natural number
* Finite verb, a verb form that has a subject, usually being inflected or marke ...

alphabet
An alphabet is a standardized set of basic written graphemes (called letters) that represent the phonemes of certain spoken languages. Not all writing systems represent language in this way; in a syllabary, each character represents a syllab ...

are of particular interest in theoretical computer science
computer science (TCS) is a subset of general computer science and mathematics that focuses on mathematical aspects of computer science such as the theory of computation, lambda calculus, and type theory.
It is difficult to circumscribe the ...

. They are often referred to simply as ''sequences'' or '' streams'', as opposed to finite '' strings''. Infinite binary sequences, for instance, are infinite sequences of bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

s (characters drawn from the alphabet ). The set ''C'' = Cantor space In mathematics, a Cantor space, named for Georg Cantor, is a topological abstraction of the classical Cantor set: a topological space is a Cantor space if it is homeomorphic to the Cantor set. In set theory, the topological space 2ω is called "the ...

.
An infinite binary sequence can represent a formal language
In logic, mathematics, computer science, and linguistics, a formal language consists of words whose letters are taken from an alphabet and are well-formed according to a specific set of rules.
The alphabet of a formal language consists of sy ...

(a set of strings) by setting the ''n'' th bit of the sequence to 1 if and only if the ''n'' th string (in shortlex order) is in the language. This representation is useful in the diagonalization method for proofs.
See also

*Enumeration
An enumeration is a complete, ordered listing of all the items in a collection. The term is commonly used in mathematics and computer science to refer to a listing of all of the elements of a set. The precise requirements for an enumeration (fo ...

* On-Line Encyclopedia of Integer Sequences
The On-Line Encyclopedia of Integer Sequences (OEIS) is an online database of integer sequences. It was created and maintained by Neil Sloane while researching at AT&T Labs. He transferred the intellectual property and hosting of the OEIS to t ...

* Recurrence relation
In mathematics, a recurrence relation is an equation according to which the nth term of a sequence of numbers is equal to some combination of the previous terms. Often, only k previous terms of the sequence appear in the equation, for a parameter ...

* Sequence space
In functional analysis and related areas of mathematics, a sequence space is a vector space whose elements are infinite sequences of real or complex numbers. Equivalently, it is a function space whose elements are functions from the natural nu ...

;Operations
* Cauchy product In mathematics, more specifically in mathematical analysis, the Cauchy product is the discrete convolution of two infinite series. It is named after the French mathematician Augustin-Louis Cauchy.
Definitions
The Cauchy product may apply to infini ...

;Examples
* Discrete-time signal
* Farey sequence
In mathematics, the Farey sequence of order ''n'' is the sequence of completely reduced fractions, either between 0 and 1, or without this restriction, which when in lowest terms have denominators less than or equal to ''n'', arranged in ord ...

* Fibonacci sequence
In mathematics, the Fibonacci numbers, commonly denoted , form a sequence, the Fibonacci sequence, in which each number is the sum of the two preceding ones. The sequence commonly starts from 0 and 1, although some authors start the sequence from ...

* Look-and-say sequence
In mathematics, the look-and-say sequence is the sequence of integers beginning as follows:
: 1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221, ... .
To generate a member of the sequence from the previous member, read off t ...

* Thue–Morse sequence
In mathematics, the Thue–Morse sequence, or Prouhet–Thue–Morse sequence, is the binary sequence (an infinite sequence of 0s and 1s) obtained by starting with 0 and successively appending the Boolean complement of the sequence obtained thus ...

* List of integer sequences
This is a list of notable integer sequences and their OEIS links.
General
Figurate numbers
Types of primes
Base-dependent
References
OEIS core sequences External links
Index to OEIS

{{DEFAULTSORT:OEIS sequences Integer se ...

;Types
* ±1-sequence
* {{DEFAULTSORT:OEIS sequences Integer se ...

Arithmetic progression
An arithmetic progression or arithmetic sequence () is a sequence of numbers such that the difference between the consecutive terms is constant. For instance, the sequence 5, 7, 9, 11, 13, 15, . . . is an arithmetic progression with a common differ ...

* Automatic sequence In mathematics and theoretical computer science, an automatic sequence (also called a ''k''-automatic sequence or a ''k''-recognizable sequence when one wants to indicate that the base of the numerals used is ''k'') is an infinite sequence of terms ...

* Cauchy sequence
In mathematics, a Cauchy sequence (; ), named after Augustin-Louis Cauchy, is a sequence whose elements become arbitrarily close to each other as the sequence progresses. More precisely, given any small positive distance, all but a finite numbe ...

* Constant-recursive sequence
In mathematics and theoretical computer science, a constant-recursive sequence is an infinite sequence of numbers where each number in the sequence is equal to a fixed linear combination of one or more of its immediate predecessors. A constant ...

* Geometric progression
In mathematics, a geometric progression, also known as a geometric sequence, is a sequence of non-zero numbers where each term after the first is found by multiplying the previous one by a fixed, non-zero number called the ''common ratio''. For ex ...

* Harmonic progression
* Holonomic sequence
* Regular sequence
In commutative algebra, a regular sequence is a sequence of elements of a commutative ring which are as independent as possible, in a precise sense. This is the algebraic analogue of the geometric notion of a complete intersection.
Definitions
Fo ...

* Pseudorandom binary sequence
A pseudorandom binary sequence (PRBS), pseudorandom binary code or pseudorandom bitstream is a binary sequence that, while generated with a deterministic algorithm, is difficult to predict and exhibits statistical behavior similar to a truly rand ...

* Random sequence The concept of a random sequence is essential in probability theory and statistics. The concept generally relies on the notion of a sequence of random variables and many statistical discussions begin with the words "let ''X''1,...,''Xn'' be independ ...

;Related concepts
* List (computing)
In computer science, a list or sequence is an abstract data type that represents a finite number of ordered values, where the same value may occur more than once. An instance of a list is a computer representation of the mathematical concept of ...

* Net (topology)
In mathematics, more specifically in general topology and related branches, a net or Moore–Smith sequence is a generalization of the notion of a sequence. In essence, a sequence is a function whose domain is the natural numbers. The codoma ...

(a generalization of sequences)
* Ordinal-indexed sequence
* Recursion (computer science)
* Set (mathematics)
A set is the mathematical model for a collection of different things; a set contains '' elements'' or ''members'', which can be mathematical objects of any kind: numbers, symbols, points in space, lines, other geometrical shapes, variables, or ...

* Tuple
In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...

* Permutation
Notes

References

External links

*The On-Line Encyclopedia of Integer Sequences

(free) {{Authority control Elementary mathematics *