The sum of the
reciprocal
Reciprocal may refer to:
In mathematics
* Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal''
* Reciprocal polynomial, a polynomial obtained from another pol ...
s of all
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
diverges; that is:
This was proved by
Leonhard Euler
Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries ...
in 1737, and strengthens
Euclid
Euclid (; grc-gre, Εὐκλείδης; BC) was an ancient Greek mathematician active as a geometer and logician. Considered the "father of geometry", he is chiefly known for the '' Elements'' treatise, which established the foundations of ...
's 3rd-century-BC result that
there are infinitely many prime numbers and
Nicole Oresme's 14th-century proof of the
divergence of the sum of the reciprocals of the integers (harmonic series).
There are a variety of proofs of Euler's result, including a
lower bound
In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is greater than or equal to every element of .
Dually, a lower bound or minorant of is defined to be an elemen ...
for the partial sums stating that
for all natural numbers . The double
natural logarithm
The natural logarithm of a number is its logarithm to the base of the mathematical constant , which is an irrational and transcendental number approximately equal to . The natural logarithm of is generally written as , , or sometimes, if ...
() indicates that the divergence might be very slow, which is indeed the case. See
Meissel–Mertens constant
The Meissel–Mertens constant (named after Ernst Meissel and Franz Mertens), also referred to as Mertens constant, Kronecker's constant, Hadamard– de la Vallée-Poussin constant or the prime reciprocal constant, is a mathematical constant in n ...
.
The harmonic series
First, we describe how Euler originally discovered the result. He was considering the
harmonic series
He had already used the following "
product formula" to show the existence of infinitely many primes.
Here the product is taken over the set of all primes.
Such infinite products are today called
Euler product In number theory, an Euler product is an expansion of a Dirichlet series into an infinite product indexed by prime numbers. The original such product was given for the sum of all positive integers raised to a certain power as proven by Leonhard Eu ...
s. The product above is a reflection of the
fundamental theorem of arithmetic. Euler noted that if there were only a finite number of primes, then the product on the right would clearly converge, contradicting the divergence of the harmonic series.
Proofs
Euler's proof
Euler considered the above product formula and proceeded to make a sequence of audacious leaps of logic. First, he took the natural logarithm of each side, then he used the Taylor series expansion for as well as the sum of a converging series:
for a fixed constant . Then he invoked the relation
which he explained, for instance in a later 1748 work, by setting in the Taylor series expansion
This allowed him to conclude that
It is almost certain that Euler meant that the sum of the reciprocals of the primes less than is asymptotic to as approaches infinity. It turns out this is indeed the case, and a more precise version of this fact was rigorously proved by
Franz Mertens
Franz Mertens (20 March 1840 – 5 March 1927) (also known as Franciszek Mertens) was a Polish mathematician. He was born in Schroda in the Grand Duchy of Posen, Kingdom of Prussia (now Środa Wielkopolska, Poland) and died in Vienna, Austria. ...
in 1874. Thus Euler obtained a correct result by questionable means.
Erdős's proof by upper and lower estimates
The following
proof by contradiction
In logic and mathematics, proof by contradiction is a form of proof that establishes the truth or the validity of a proposition, by showing that assuming the proposition to be false leads to a contradiction. Proof by contradiction is also known ...
comes from
Paul Erdős
Paul Erdős ( hu, Erdős Pál ; 26 March 1913 – 20 September 1996) was a Hungarian mathematician. He was one of the most prolific mathematicians and producers of mathematical conjectures of the 20th century. pursued and proposed problems in ...
.
Let denote the th prime number. Assume that the
sum of the reciprocals of the primes
converges.
Then there exists a smallest
positive integer
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 languag ...
such that
For a positive integer , let denote the set of those in which are not
divisible by any prime greater than (or equivalently all which are a product of powers of primes ). We will now derive an upper and a lower estimate for , the
number of elements in . For large , these bounds will turn out to be contradictory.
;Upper estimate:
:Every in can be written as with positive integers and , where is
square-free. Since only the primes can show up (with exponent 1) in the
prime factorization
In number theory, integer factorization is the decomposition of a composite number into a product of smaller integers. If these factors are further restricted to prime numbers, the process is called prime factorization.
When the numbers are s ...
of , there are at most different possibilities for . Furthermore, there are at most possible values for . This gives us the upper estimate
;Lower estimate:
:The remaining numbers in the
set difference
In set theory, the complement of a set , often denoted by (or ), is the set of elements not in .
When all sets in the universe, i.e. all sets under consideration, are considered to be members of a given set , the absolute complement of is th ...
are all divisible by a prime greater than . Let denote the set of those in which are divisible by the th prime . Then
:Since the number of integers in is at most (actually zero for ), we get
:Using (1), this implies
This produces a contradiction: when , the estimates (2) and (3) cannot both hold, because .
Proof that the series exhibits log-log growth
Here is another proof that actually gives a lower estimate for the partial sums; in particular, it shows that these sums grow at least as fast as . The proof is due to Ivan Niven, adapted from the product expansion idea of
Euler
Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ...
. In the following, a sum or product taken over always represents a sum or product taken over a specified set of primes.
The proof rests upon the following four inequalities:
* Every positive integer can be uniquely expressed as the product of a square-free integer and a square as a consequence of the
fundamental theorem of arithmetic. Start with
where the ''β''s are 0 (the corresponding power of prime is even) or 1 (the corresponding power of prime is odd). Factor out one copy of all the primes whose β is 1, leaving a product of primes to even powers, itself a square. Relabeling:
where the first factor, a product of primes to the first power, is square free. Inverting all the s gives the inequality
To see this, note that
and
That is,
is one of the summands in the expanded product . And since
is one of the summands of , every summand
is represented in one of the terms of when multiplied out. The inequality follows.
* The upper estimate for the
natural logarithm
The natural logarithm of a number is its logarithm to the base of the mathematical constant , which is an irrational and transcendental number approximately equal to . The natural logarithm of is generally written as , , or sometimes, if ...
Combining all these inequalities, we see that
Dividing through by and taking the natural logarithm of both sides gives
as desired.
Q.E.D.
Q.E.D. or QED is an initialism of the Latin phrase , meaning "which was to be demonstrated". Literally it states "what was to be shown". Traditionally, the abbreviation is placed at the end of mathematical proofs and philosophical arguments in pri ...
Using
(see the
Basel problem), the above constant can be improved to ; in fact it turns out that
where is the
Meissel–Mertens constant
The Meissel–Mertens constant (named after Ernst Meissel and Franz Mertens), also referred to as Mertens constant, Kronecker's constant, Hadamard– de la Vallée-Poussin constant or the prime reciprocal constant, is a mathematical constant in n ...
(somewhat analogous to the much more famous
Euler–Mascheroni constant
Euler's constant (sometimes also called the Euler–Mascheroni constant) is a mathematical constant usually denoted by the lowercase Greek letter gamma ().
It is defined as the limiting difference between the harmonic series and the natural l ...
).
Proof from Dusart's inequality
From
Dusart's inequality, we get
Then
by the
integral test for convergence. This shows that the series on the left diverges.
Geometric and harmonic-series proof
Suppose for contradiction the sum converged. Then, there exists
such that
. Call this sum
.
Now consider the convergent geometric series
.
This geometric series contains the sum of reciprocals of all numbers whose prime factorization contain only primes in the set
.
Consider the subseries
. This is a subseries because
is not divisible by any
.
However, by the
Limit comparison test, this subseries diverges by comparing it to the harmonic series. Indeed,
.
Thus, we have found a divergent subseries of the original convergent series, and since all terms are positive, this gives the contradiction. We may conclude
diverges. Q.E.D.
Partial sums
While the
partial sum
In mathematics, a series is, roughly speaking, a description of the operation of adding infinitely many quantities, one after the other, to a given starting quantity. The study of series is a major part of calculus and its generalization, math ...
s of the reciprocals of the primes eventually exceed any integer value, they never equal an integer.
One proof
is by induction: The first partial sum is , which has the form . If the th partial sum (for ) has the form , then the st sum is
as the st prime is odd; since this sum also has an form, this partial sum cannot be an integer (because 2 divides the denominator but not the numerator), and the induction continues.
Another proof rewrites the expression for the sum of the first reciprocals of primes (or indeed the sum of the reciprocals of ''any'' set of primes) in terms of the
least common denominator, which is the product of all these primes. Then each of these primes divides all but one of the numerator terms and hence does not divide the numerator itself; but each prime ''does'' divide the denominator. Thus the expression is irreducible and is non-integer.
See also
*
Euclid's theorem that there are infinitely many primes
*
Small set (combinatorics)
*
Brun's theorem, on the convergent sum of reciprocals of the twin primes
*
List of sums of reciprocals
References
Sources
*
External links
*
{{DEFAULTSORT:Sum Of The Reciprocals Of The Primes Diverges
Mathematical series
Articles containing proofs
Theorems about prime numbers