free abelian group

TheInfoList

In
mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It has no generally ...
, a free abelian group is an
abelian group In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
with a
basis Basis may refer to: Finance and accounting *Adjusted basisIn tax accounting, adjusted basis is the net cost of an asset after adjusting for various tax-related items. Adjusted Basis or Adjusted Tax Basis refers to the original cost or other ba ...

. Being an abelian group means that it is a set with an addition operation that is
associative In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
,
commutative In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
, and invertible. A basis, also called an integral basis, is a subset such that every element of the group can be uniquely expressed as an
integer An integer (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of t ...
combination In mathematics, a combination is a selection of items from a collection, such that the order of selection does not matter (unlike permutations). For example, given three fruits, say an apple, an orange and a pear, there are three combinations of t ...
of finitely many basis elements. For instance the two-dimensional
integer latticeIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
forms a free abelian group, with coordinatewise addition as its operation, and with the two points (1,0) and (0,1) as its basis. Free abelian groups have properties which make them similar to
vector space In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s, and may equivalently be called free the
free module In mathematics, a free module is a module (mathematics), module that has a Basis (linear algebra), basis – that is, a generating set of a module, generating set consisting of linearly independent elements. Every vector space is a free module, but, ...

s over the integers.
Lattice theory A lattice is an abstract structure studied in the mathematical subdisciplines of order theory and abstract algebra. It consists of a partially ordered set in which every two elements have a unique supremum (also called a least upper boun ...
studies free abelian
subgroup In group theory The popular puzzle Rubik's cube invented in 1974 by Ernő Rubik has been used as an illustration of permutation group">Ernő_Rubik.html" ;"title="Rubik's cube invented in 1974 by Ernő Rubik">Rubik's cube invented in 1974 by ...
s of real vector spaces. In
algebraic topology 250px, A torus, one of the most frequently studied objects in algebraic topology Algebraic topology is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathemat ...
, free abelian groups are used to define chain groups, and in
algebraic geometry Algebraic geometry is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (ma ...

they are used to define
divisors 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 by another integer m if m ...
. The elements of a free abelian group with basis $B$ may be described in several equivalent ways. These include formal sums which are expressions of the form $\sum a_i b_i$ where each $a_i$ is a nonzero integer, each $b_i$ is a distinct basis element, and the sum has finitely many terms. Alternatively, the elements of a free abelian group may be thought of as signed
multiset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s containing finitely many elements with the multiplicity of an element in the multiset equal to its coefficient in the formal sum. Another way to represent an element of a free abelian group is as a function from $B$ to the integers with finitely many nonzero values; for this functional representation, the group operation is the
pointwiseIn 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 o ...
addition of functions. Every set $B$ has a free abelian group with $B$ as its basis. This group is unique in the sense that every two free abelian groups with the same basis are
isomorphic In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
. Instead of constructing it by describing its individual elements, a
free group for the free group on two generators would look like. Each vertex represents an element of the free group, and each edge represents multiplication by ''a'' or ''b''. In mathematics Mathematics (from Ancient Greek, Greek: ) includes the stud ...
with basis $B$ may be constructed as a
direct sum The direct sum is an operation from abstract algebra, a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geome ...
of copies of the additive group of the integers, with one copy per member Alternatively, the free abelian group with basis $B$ may be described by a
presentation A presentation conveys information from a speaker to an audience An audience is a group of people who participate in a show or encounter a work of art A work of art, artwork, art piece, piece of art or art object is an a ...
with the elements of $B$ as its generators and with the
commutator In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s of pairs of members as its relators. The
rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as: Level or position in a hierarchical organization * Academic rank * Diplomatic rank * Hierarchy * Hi ...
of a free abelian group is the
cardinality In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
of a basis; every two bases for the same group give the same rank, and every two free abelian groups with the same rank are isomorphic. Every subgroup of a free abelian group is itself free abelian; this fact allows a general abelian group to be understood as a
quotient In arithmetic Arithmetic (from the Ancient Greek, Greek wikt:en:ἀριθμός#Ancient Greek, ἀριθμός ''arithmos'', 'number' and wikt:en:τική#Ancient Greek, τική wikt:en:τέχνη#Ancient Greek, έχνη ''tiké échne', ...
of a free abelian group by "relations", or as a
cokernel The cokernel of a linear mapping of vector spaces is the quotient space of the codomain of by the image of . The dimension of the cokernel is called the ''corank'' of . Cokernels are dual to the kernels of category theory, hence the nam ...

of an injective
homomorphism In algebra, a homomorphism is a morphism, structure-preserving map (mathematics), map between two algebraic structures of the same type (such as two group (mathematics), groups, two ring (mathematics), rings, or two vector spaces). The word ''homom ...

between free abelian groups. The only free abelian groups that are free groups are the
trivial groupIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
and the
infinite cyclic group In group theory The popular puzzle Rubik's cube invented in 1974 by Ernő Rubik has been used as an illustration of permutation group">Ernő_Rubik.html" ;"title="Rubik's cube invented in 1974 by Ernő Rubik">Rubik's cube invented in 1974 by Er ...
.

# Definition and examples

A free abelian group is an
abelian group In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
that has a basis. Here, being an abelian group means that it is described by a set $S$ of its elements and a
binary operation In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
conventionally denoted as an
additive group An additive group is a group of which the group operation is to be thought of as ''addition'' in some sense. It is usually abelian, and typically written using the symbol + for its binary operation. This terminology is widely used with structure ...
by the $+$ symbol (although it need not be the usual addition of numbers) that obey the following properties: *The operation $+$ is
commutative In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
and
associative In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
, meaning for all elements $x+y=y+x$ and Therefore, when combining two or more elements of $S$ using this operation, the ordering and grouping of the elements does not affect the result. *$S$ contains an
identity element In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
(conventionally denoted with the property that, for every *Every element $x$ in $S$ has an
inverse element In abstract algebra, the idea of an inverse element generalises the concepts of negation (sign reversal) (in relation to addition Addition (usually signified by the plus symbol ) is one of the four basic operations of arithmetic Arithmetic ...
such that A basis is a subset $B$ of the elements of $S$ with the property that every element of $S$ may be formed in a unique way by choosing finitely many basis elements $b_i$ choosing a nonzero integer $k_i$ for each of the chosen basis elements, and adding together $k_i$ copies of the basis elements $b_i$ for which $k_i$ is positive, and $-k_i$ copies of $-b_i$ for each basis element for which $k_i$ is negative. As a special case, the identity element can always be formed in this way as the combination of zero basis elements, according to the usual convention for an
empty sum In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
, and it must not be possible to find any other combination that represents the identity. The under the usual addition operation, form a free abelian group with the The integers are commutative and associative, with as the
additive identityIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
and with each integer having an
additive inverse In mathematics, the additive inverse of a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) ...
, its negation. Each non-negative $x$ is the sum of $x$ copies and each negative integer $x$ is the sum of $-x$ copies so the basis property is also satisfied. An example where the group operation is different from the usual addition of numbers is given by the positive
rational number In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s which form a free abelian group with the usual
multiplication Multiplication (often denoted by the cross symbol , by the mid-line dot operator , by juxtaposition, or, on computers, by an asterisk ) is one of the four elementary Operation (mathematics), mathematical operations of arithmetic, with the ...

operation on numbers and with the
prime number A prime number (or a prime) is a natural number greater than 1 that is not a Product (mathematics), 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 ...
s as their basis. Multiplication is commutative and associative, with the number $1$ as its identity and with $1/x$ as the inverse element for each positive rational The fact that the prime numbers forms a basis for multiplication of these numbers follows from the
fundamental theorem of arithmetic In number theory, the fundamental theorem of arithmetic, also called the unique factorization theorem or the unique-prime-factorization theorem, states that every integer An integer (from the Latin wikt:integer#Latin, ''integer'' meaning "wh ...
, according to which every positive integer can be factorized uniquely into the product of finitely many primes or their inverses. If $q=a/b$ is a positive rational number expressed in simplest terms, then $q$ can be expressed as a finite combination of the primes appearing in the factorizations of $a$ The number of copies of each prime to use in this combination is its exponent in the factorization of $a$, or the negation of its exponent in the factorization The
polynomial In mathematics, a polynomial is an expression (mathematics), expression consisting of indeterminate (variable), indeterminates (also called variable (mathematics), variables) and coefficients, that involves only the operations of addition, subtra ...

s of a single with integer coefficients, form a free abelian group under polynomial addition, with the powers of $x$ as a basis. As an abstract group, this is the same as (an
isomorphic group In abstract algebra, a group isomorphism is a function between two groups that sets up a one-to-one correspondence between the elements of the groups in a way that respects the given group operations. If there exists an isomorphism between two gr ...
to) the multiplicative group of positive rational numbers. One way to map these two groups to each other, showing that they are isomorphic, is to reinterpret the exponent of the prime number in the multiplicative group of the rationals as instead giving the coefficient of $x^$ in the corresponding polynomial, or vice versa. Although the representation of each group element in terms of a given basis is unique, a free abelian group has generally more than one basis, and different bases will generally result in different representations of its elements. For example, if one replaces any element of a basis by its negation, one gets another basis. As a more elaborated example, the two-dimensional
integer latticeIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
consisting of the points in the plane with integer
Cartesian coordinates A Cartesian coordinate system (, ) in a plane Plane or planes may refer to: * Airplane or aeroplane or informally plane, a powered, fixed-wing aircraft Arts, entertainment and media *Plane (Dungeons & Dragons), Plane (''Dungeons & Dragons'') ...

, forms a free abelian group under
vector addition In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...

with the basis For this basis, the element $\left(4,3\right)$ can be written where 'multiplication' is defined so that, for instance, There is no other way to write $\left(4,3\right)$ in the same basis. However, with a different basis such as it can be written as Generalizing this example, every lattice forms a finitely-generated free abelian group. The integer lattice $\Z^d$ has a natural basis consisting of the positive integer
unit vector In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s, but it has many other bases as well: if $M$ is a $d\times d$ integer matrix with then the rows of $M$ form a basis, and conversely every basis of the integer lattice has this form. For more on the two-dimensional case, see fundamental pair of periods.

# Constructions

Every set can be the basis of a free abelian group, which is unique up to group isomorphisms. The free abelian group for a given basis set can be constructed in several different but equivalent ways: as a direct sum of copies of the integers, as a family of integer-valued functions, as a signed multiset, or by
presentation of a group In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
.

## Products and sums

The
direct product of groups In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
consists of tuples of an element from each group in the product, with pointwise addition. The direct product of two free abelian groups is itself free abelian, with basis the
disjoint union In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

of the bases of the two groups. More generally the direct product of any finite number of free abelian groups is free abelian. The integer lattice, for instance, is isomorphic to the direct product of $d$ copies of the integer The trivial group $\$ is also considered to be free abelian, with basis the
empty set#REDIRECT Empty set In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical an ...

. It may be interpreted as an
empty product In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
, the direct product of zero copies For infinite families of free abelian groups, the direct product is not necessarily free abelian. For instance the
Baer–Specker groupIn mathematics, in the field of group theory, the Baer–Specker group, or Specker group, named after Reinhold Baer and Ernst Specker, is an example of an infinite Abelian group which is a building block in the structure theory of such groups. Defin ...
an uncountable group formed as the direct product of
countably In mathematics, a countable set is a Set (mathematics), set with the same cardinality (cardinal number, number of elements) as some subset of the set of natural numbers. A countable set is either a finite set or a countably infinite set. Whether f ...
many copies was shown in 1937 by
Reinhold Baer Reinhold Baer (22 July 1902 – 22 October 1979) was a Germany, German mathematician, known for his work in algebra. He introduced injective modules in 1940. He is the eponym of Baer rings and Baer groups. Biography Baer studied mechanical engine ...
to not be free abelian, although Ernst Specker proved in 1950 that all of its countable subgroups are free abelian. Instead, to obtain a free abelian group from an infinite family of groups, the direct sum of groups, direct sum rather than the direct product should be used. The direct sum and direct product are the same when they are applied to finitely many groups, but differ on infinite families of groups. In the direct sum, the elements are again tuples of elements from each group, but with the restriction that all but finitely many of these elements are the identity for their group. The direct sum of infinitely many free abelian groups remains free abelian. It has a basis consisting of tuples in which all but one element is the identity, with the remaining element part of a basis for its group. Every free abelian group may be described as a direct sum of copies with one copy for each member of its basis. This construction allows any set $B$ to become the basis of a free abelian group.

## Integer functions and formal sums

Given a one can define a group $\mathbb^$ whose elements are functions from $B$ to the integers, where the parenthesis in the superscript indicates that only the functions with finitely many nonzero values are included. If $f\left(x\right)$ and $g\left(x\right)$ are two such functions, then $f+g$ is the function whose values are sums of the values in $f$ that is, This
pointwiseIn 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 o ...
addition operation gives $\mathbb^$ the structure of an abelian group. Each element $x$ from the given set $B$ corresponds to a member the function $e_x$ for which $e_x\left(x\right)=1$ and for which $e_x\left(y\right)=0$ for Every function $f$ in $\mathbb^$ is uniquely a linear combination of a finite number of basis elements: $f=\sum_ f(x) e_x.$ Thus, these elements $e_x$ form a basis and $\mathbb^$ is a free abelian group. In this way, every set $B$ can be made into the basis of a free abelian group. The elements of $\mathbb^$ may also be written as formal sums, expressions in the form of a sum of finitely many terms, where each term is written as the product of a nonzero integer with a distinct member These expressions are considered equivalent when they have the same terms, regardless of the ordering of terms, and they may be added by forming the union of the terms, adding the integer coefficients to combine terms with the same basis element, and removing terms for which this combination produces a zero coefficient. They may also be interpreted as the signed
multiset In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s of finitely many elements

## Presentation

A
presentation of a group In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). I ...
is a set of elements that generate the group (meaning that all group elements can be expressed as products of finitely many generators), together with "relators", products of generators that give the identity element. The free abelian group with basis $B$ has a presentation in which the generators are the elements and the relators are the
commutator In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
s of pairs of elements Here, the commutator of two elements $x$ and $y$ is the product setting this product to the identity causes $xy$ to so that $x$ and $y$ commute. More generally, if all pairs of generators commute, then all pairs of products of generators also commute. Therefore, the group generated by this presentation is abelian, and the relators of the presentation form a minimal set of relators needed to ensure that it is abelian. When the set of generators is finite, the presentation of a free abelian group is also finite, because there are only finitely many different commutators to include in the presentation. This fact, together with the fact that every subgroup of a free abelian group is free abelian (#Subgroups, below) can be used to show that every finitely generated abelian group is finitely presented. For, if $G$ is finitely generated by a it is a quotient of the free abelian group over $B$ by a free abelian subgroup, the subgroup generated by the relators of the presentation But since this subgroup is itself free abelian, it is also finitely generated, and its basis (together with the commutators forms a finite set of relators for a presentation

# As a module

The Module (mathematics), modules over the integers are defined similarly to
vector space In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s over the real numbers or
rational number In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s: they consist of systems of elements that can be added to each other, with an operation for scalar multiplication by integers that is compatible with this addition operation. Every abelian group may be considered as a module over the integers, with a scalar multiplication operation defined as follows: However, unlike vector spaces, not all abelian groups have a basis, hence the special name for those that do. A
free module In mathematics, a free module is a module (mathematics), module that has a Basis (linear algebra), basis – that is, a generating set of a module, generating set consisting of linearly independent elements. Every vector space is a free module, but, ...

is a module that can be represented as a direct sum over its base ring, so free abelian groups and free are equivalent concepts: each free abelian group is (with the multiplication operation above) a free and each free comes from a free abelian group in this way. As well as the direct sum, another way to combine free abelian groups is to use the tensor product of modules, tensor product of The tensor product of two free abelian groups is always free abelian, with a basis that is the Cartesian product of the bases for the two groups in the product. Many important properties of free abelian groups may be generalized to free modules over a principal ideal domain. For instance, submodules of free modules over principal ideal domains are free, a fact that writes allows for "automatic generalization" of homological machinery to these modules. Additionally, the theorem that every projective is free generalizes in the same way.

# Properties

## Universal property

A free abelian group $F$ with basis $B$ has the following universal property: for every function $f$ from $B$ to an abelian group $A$, there exists a unique group homomorphism from $F$ to $A$ which extends $f$. By a general property of universal properties, this shows that "the" abelian group of base $B$ is unique up to an isomorphism. Therefore, the universal property can be used as a definition of the free abelian group of base $B$. The uniqueness of the group defined by this property shows that all the other definitions are equivalent. It is because of this universal property that free abelian groups are called "free": they are the free objects in the category of abelian groups, and the map from a basis to its free abelian group is a functor from sets to abelian groups, Adjoint functors, adjoint to the forgetful functor from abelian groups to sets. However, a ''free abelian'' group is ''not'' a
free group for the free group on two generators would look like. Each vertex represents an element of the free group, and each edge represents multiplication by ''a'' or ''b''. In mathematics Mathematics (from Ancient Greek, Greek: ) includes the stud ...
except in two cases: a free abelian group having an empty basis (rank zero, giving the
trivial groupIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
) or having just one element in the basis (rank one, giving the
infinite cyclic group In group theory The popular puzzle Rubik's cube invented in 1974 by Ernő Rubik has been used as an illustration of permutation group">Ernő_Rubik.html" ;"title="Rubik's cube invented in 1974 by Ernő Rubik">Rubik's cube invented in 1974 by Er ...
). Other abelian groups are not free groups because in free groups $ab$ must be different from $ba$ if $a$ and $b$ are different elements of the basis, while in free abelian groups the two products must be identical for all pairs of elements. In the general category of groups, it is an added constraint to demand that $ab=ba$, whereas this is a necessary property in the category of abelian groups.

## Rank

Every two bases of the same free abelian group have the same
cardinality In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
, so the cardinality of a basis forms an Invariant (mathematics), invariant of the group known as its
rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as: Level or position in a hierarchical organization * Academic rank * Diplomatic rank * Hierarchy * Hi ...
. Two free abelian groups are isomorphic if and only if they have the same rank. A free abelian group is finitely generated module, finitely generated if and only if its rank is a finite number $n$, in which case the group is isomorphic to $\mathbb^n$. This notion of rank can be generalized, from free abelian groups to abelian groups that are not necessarily free. The rank of an abelian group $G$ is defined as the rank of a free abelian subgroup $F$ of $G$ for which the quotient group $G/F$ is a torsion group. Equivalently, it is the cardinality of a maximal element, maximal subset of $G$ that generates a free subgroup. Again, this is a group invariant; it does not depend on the choice of the subgroup.

## Subgroups

Every subgroup of a free abelian group is itself a free abelian group. This result of Richard Dedekind was a precursor to the analogous Nielsen–Schreier theorem that every subgroup of a
free group for the free group on two generators would look like. Each vertex represents an element of the free group, and each edge represents multiplication by ''a'' or ''b''. In mathematics Mathematics (from Ancient Greek, Greek: ) includes the stud ...
is free, and is a generalization of the fact that Subgroups of cyclic groups, every nontrivial subgroup of the infinite cyclic group is infinite cyclic. The proof needs the axiom of choice. A proof using Zorn's lemma (one of many equivalent assumptions to the axiom of choice) can be found in Serge Lang's ''Algebra''. Solomon Lefschetz and Irving Kaplansky have claimed that using the well-ordering principle in place of Zorn's lemma leads to a more intuitive proof. In the case of finitely generated free abelian groups, the proof is easier, does not need the axiom of choice, and leads to a more precise result. If $G$ is a subgroup of a finitely generated free abelian group $F$, then $G$ is free and there exists a basis $\left(e_1, \ldots, e_n\right)$ of $F$ and positive integers $d_1, d_2, \ldots, d_k$ (that is, each one divides the next one) such that $\left(d_1e_1,\ldots, d_ke_k\right)$ is a basis of $G.$ Moreover, the sequence $d_1,d_2,\ldots,d_k$ depends only on $F$ and $G$ and not on the particular basis $\left(e_1, \ldots, e_n\right)$ that solves the problem. A constructive proof of the existence part of the theorem is provided by any algorithm computing the Smith normal form of a matrix of integers. Uniqueness follows from the fact that, for any $r\le k$, the greatest common divisor of the minors of rank $r$ of the matrix is not changed during the Smith normal form computation and is the product $d_1\cdots d_r$ at the end of the computation.

## Torsion and divisibility

All free abelian groups are torsion (algebra), torsion-free, meaning that there is no group element (non-identity) $x$ and nonzero integer $n$ such that $nx=0$. Conversely, all finitely generated torsion-free abelian groups are free abelian. The additive group of
rational number In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...
s $\mathbb$ provides an example of a torsion-free (but not finitely generated) abelian group that is not free abelian. One reason that $\mathbb$ is not free abelian is that it is divisible group, divisible, meaning that, for every element $x\in\mathbb$ and every nonzero integer $n$, it is possible to express $x$ as a scalar multiple $ny$ of another element $y=x/n$. In contrast, non-trivial free abelian groups are never divisible, because in a free abelian group the basis elements cannot be expressed as multiples of other elements.

## Symmetry

The symmetries of any group can be described as group automorphisms, the bijection, invertible homomorphisms from the group to itself. In non-abelian groups these are further subdivided into inner and outer automorphisms, but in abelian groups all non-identity automorphisms are outer. They form another group, the automorphism group of the given group, under the operation of Function composition, composition. The automorphism group of a free abelian group of finite rank $n$ is the general linear group $GL\left(n,\mathbb\right)$, which can be described concretely (for a specific basis of the free automorphism group) as the set of $n\times n$ invertible integer matrix (mathematics), matrices under the operation of matrix multiplication. Their action as symmetries on the free abelian group $\Z^n$ is just matrix-vector multiplication. The automorphism groups of two infinite-rank free abelian groups have the same first-order theory, first-order theories as each other, if and only if their ranks are equivalent cardinals from the point of view of second-order logic. This result depends on the structure of Involution (mathematics), involutions of free abelian groups, the automorphisms that are their own inverse. Given a basis for a free abelian group, one can find involutions that map any set of disjoint pairs of basis elements to each other, or that negate any chosen subset of basis elements, leaving the other basis elements fixed. Conversely, for every involution of a free abelian group, one can find a basis of the group for which all basis elements are swapped in pairs, negated, or left unchanged by the involution.

## Relation to other groups

If a free abelian group is a quotient of two groups $A/B$, then $A$ is the direct sum $B\oplus A/B$. Given an arbitrary abelian group $A$, there always exists a free abelian group $F$ and a surjective group homomorphism from $F$ to $A$. One way of constructing a surjection onto a given group $A$ is to let $F=\mathbb^$ be the free abelian group over $A$, represented as formal sums. Then a surjection can be defined by mapping formal sums in $F$ to the corresponding sums of members of $A$. That is, the surjection maps $\sum_ a_x e_x \mapsto \sum_ a_x x,$ where $a_x$ is the integer coefficient of basis element $e_x$ in a given formal sum, the first sum is in $F$, and the second sum is in $A$. This surjection is the unique group homomorphism which extends the function $e_x\mapsto x$, and so its construction can be seen as an instance of the universal property. When $F$ and $A$ are as above, the Kernel (algebra), kernel $G$ of the surjection from $F$ to $A$ is also free abelian, as it is a subgroup of $F$ (the subgroup of elements mapped to the identity). Therefore, these groups form a short exact sequence $0\to G\to F\to A\to 0$ in which $F$ and $G$ are both free abelian and $A$ is isomorphic to the factor group $F/G$. This is a free resolution of $A$. Furthermore, assuming the axiom of choice, the free abelian groups are precisely the projective module, projective objects in the category of abelian groups.

# Applications

## Algebraic topology

In
algebraic topology 250px, A torus, one of the most frequently studied objects in algebraic topology Algebraic topology is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathemat ...
, a formal sum of $k$-dimensional simplex, simplices is called a $k$-chain, and the free abelian group having a collection of $k$-simplices as its basis is called a chain group. The simplices are generally taken from some topological space, for instance as the set of $k$-simplices in a simplicial complex, or the set of singular homology, singular $k$-simplices in a manifold. Any $k$-dimensional simplex has a boundary that can be represented as a formal sum of $\left(k-1\right)$-dimensional simplices, and the universal property of free abelian groups allows this boundary operator to be extended to a group homomorphism from $k$-chains to $\left(k-1\right)$-chains. The system of chain groups linked by boundary operators in this way forms a chain complex, and the study of chain complexes forms the basis of homology theory.

## Algebraic geometry and complex analysis

Every rational function over the complex numbers can be associated with a signed multiset of complex numbers $c_i$, the zeros and poles of the function (points where its value is zero or infinite). The multiplicity $m_i$ of a point in this multiset is its order as a zero of the function, or the negation of its order as a pole. Then the function itself can be recovered from this data, up to a Scalar (mathematics), scalar factor, as $f(q)=\prod (q-c_i)^.$ If these multisets are interpreted as members of a free abelian group over the complex numbers, then the product or quotient of two rational functions corresponds to the sum or difference of two group members. Thus, the multiplicative group of rational functions can be factored into the multiplicative group of complex numbers (the associated scalar factors for each function) and the free abelian group over the complex numbers. The rational functions that have a nonzero limiting value at infinity (the meromorphic functions on the Riemann sphere) form a subgroup of this group in which the sum of the multiplicities is zero. This construction has been generalized, in
algebraic geometry Algebraic geometry is a branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (ma ...

, to the notion of a Divisor (algebraic geometry), divisor. There are different definitions of divisors, but in general they form an abstraction of a codimension-one subvariety of an algebraic variety, the set of solution points of a system of polynomial equations. In the case where the system of equations has one degree of freedom (its solutions form an algebraic curve or Riemann surface), a subvariety has codimension one when it consists of isolated points, and in this case a divisor is again a signed multiset of points from the variety. The meromorphic functions on a compact Riemann surface have finitely many zeros and poles, and their divisors can again be represented as elements of a free abelian group, with multiplication or division of functions corresponding to addition or subtraction of group elements. However, in this case there are additional constraints on the divisor beyond having zero sum of multiplicities.

## Group rings

The group ring $\Z\left[G\right]$, for any group $G$, is ring whose additive group is the free abelian group over $G$. When $G$ is finite and abelian, the multiplicative group of Unit (ring theory), units in $\Z\left[G\right]$ has the structure of a direct product of a finite group and a finitely generated free abelian group.

# References

{{DEFAULTSORT:Free Abelian Group Abelian group theory Properties of groups Free algebraic structures