# Euclid's theorem

**Euclid's theorem** is a fundamental statement in number theory that asserts that there are infinitely many prime numbers. It was first proved by Euclid in his work *Elements*. There are several proofs of the theorem.

Field | Number theory |
---|---|

First proof by | Euclid |

First proof in | c. 300 BCE |

Generalizations | Dirichlet's theorem on arithmetic progressions Prime number theorem |

## Euclid's proof

Euclid offered a proof published in his work *Elements* (Book IX, Proposition 20),[1] which is paraphrased here.[2]

Consider any finite list of prime numbers *p*_{1}, *p*_{2}, ..., *p*_{n}. It will be shown that at least one additional prime number not in this list exists. Let *P* be the product of all the prime numbers in the list: *P* = *p*_{1}*p*_{2}...*p*_{n}. Let *q* = *P* + 1. Then *q* is either prime or not:

- If
*q*is prime, then there is at least one more prime that is not in the list. - If
*q*is not prime, then some prime factor*p*divides*q*. If this factor*p*were in our list, then it would divide*P*(since*P*is the product of every number in the list); but*p*divides*P*+ 1 =*q*. If*p*divides*P*and*q,*then*p*would have to divide the difference[3] of the two numbers, which is (*P*+ 1) −*P*or just 1. Since no prime number divides 1,*p*cannot be on the list. This means that at least one more prime number exists beyond those in the list.

This proves that for every finite list of prime numbers there is a prime number not in the list.[4] In the original work, as Euclid had no way of writing an arbitrary list of primes, he used a method that he frequently applied, that is, the method of generalizable example. Namely, he picks just three primes and using the general method outlined above, proves that he can always find an additional prime. Euclid presumably assumes that his readers are convinced that a similar proof will work, no matter how many primes are originally picked.[5]

Euclid is often erroneously reported to have proved this result by contradiction beginning with the assumption that the finite set initially considered contains all prime numbers,[6] though it is actually a proof by cases, a direct proof method. The philosopher Torkel Franzén, in a book on logic, states, "Euclid's proof that there are infinitely many primes is not an indirect proof [...] The argument is sometimes formulated as an indirect proof by replacing it with the assumption 'Suppose *q*_{1}, ... *q*_{n} are all the primes'. However, since this assumption isn't even used in the proof, the reformulation is pointless."[7]

### Variations

Several variations on Euclid's proof exist, including the following:

The factorial *n*! of a positive integer *n* is divisible by every integer from 2 to *n*, as it is the product of all of them. Hence, *n*! + 1 is not divisible by any of the integers from 2 to *n*, inclusive (it gives a remainder of 1 when divided by each). Hence *n*! + 1 is either prime or divisible by a prime larger than *n*. In either case, for every positive integer *n*, there is at least one prime bigger than *n*. The conclusion is that the number of primes is infinite.[8]

## Euler's proof

Another proof, by the Swiss mathematician Leonhard Euler, relies on the fundamental theorem of arithmetic: that every integer has a unique prime factorization. If *P* is the set of all prime numbers, Euler wrote that:

The first equality is given by the formula for a geometric series in each term of the product. The second equality is a special case of the Euler product formula for the Riemann zeta function. To show this, distribute the product over the sum:

In the result, every product of primes appears exactly once and so by the fundamental theorem of arithmetic the sum is equal to the sum over all integers.

The sum on the right is the harmonic series, which diverges. Thus the product on the left must also diverge. Since each term of the product is finite, the number of terms must be infinite; therefore, there is an infinite number of primes.

## Erdős's proof

Paul Erdős gave a third proof that also relies on the fundamental theorem of arithmetic. First every integer *n* can be uniquely written as

where *r* is *square-free*, or not divisible by any square numbers (let *s*^{2} be the largest square number that divides *n* and then let *r* = *n*/*s*^{2}). Now suppose that there are only finitely many prime numbers and call the number of prime numbers *k*. As each of the prime numbers factorizes any squarefree number at most once, by the fundamental theorem of arithmetic, there are only *2*^{k} square-free numbers (see Combination#Number of *k*-combinations for all *k*).

Now fix a positive integer *N* and consider the integers between 1 and *N*. Each of these numbers can be written as *rs*^{2} where *r* is square-free and *s*^{2} is a square, like this:

- ( 1×1, 2×1, 3×1, 1×4, 5×1, 6×1, 7×1, 2×4, 1×9, 10×1, ...)

There are *N* different numbers in the list. Each of them is made by multiplying a squarefree number, by a square number that is *N* or less. There are ⌊√*N*⌋ such square numbers. Then, we form all the possible products of all squares less than *N* multiplied by all squarefrees everywhere. There are exactly 2^{k}⌊√*N*⌋ such numbers, all different, and they include all the numbers in our list and maybe more. Therefore, 2^{k}⌊√*N*⌋ ≥ *N*. Here, ⌊*x*⌋ denotes the floor function.

Since this inequality does not hold for *N* sufficiently large, there must be infinitely many primes.

## Furstenberg's proof

In the 1950s, Hillel Furstenberg introduced a proof by contradiction using point-set topology.[9]

Define a topology on the integers **Z**, called the evenly spaced integer topology, by declaring a subset *U* ⊆ **Z** to be an open set if and only if it is either the empty set, ∅, or it is a union of arithmetic sequences *S*(*a*, *b*) (for *a* ≠ 0), where

Then a contradiction follows from the property that a finite set of integers cannot be open and the property that the basis sets *S*(*a*, *b*) are both open and closed, since

cannot be closed because its complement is finite, but is closed since it is a finite union of closed sets.

## Some recent proofs

### Proof using the inclusion-exclusion principle

Juan Pablo Pinasco has written the following proof.[10]

Let *p*_{1}, ..., *p*_{N} be the smallest *N* primes. Then by the inclusion–exclusion principle, the number of positive integers less than or equal to *x* that are divisible by one of those primes is

Dividing by *x* and letting *x* → ∞ gives

This can be written as

If no other primes than *p*_{1}, ..., *p*_{N} exist, then the expression in (1) is equal to and the expression in (2) is equal to 1, but clearly the expression in (3) is not equal to 1. Therefore, there must be more primes than *p*_{1}, ..., *p*_{N}.

### Proof using de Polignac's formula

In 2010, Junho Peter Whang published the following proof by contradiction.[11] Let *k* be any positive integer. Then according to de Polignac's formula (actually due to Legendre)

where

But if only finitely many primes exist, then

(the numerator of the fraction would grow singly exponentially while by Stirling's approximation the denominator grows more quickly than singly exponentially),
contradicting the fact that for each *k* the numerator is greater than or equal to the denominator.

### Proof by construction

Filip Saidak gave the following proof by construction, which does not use reductio ad absurdum[12] or Euclid's Lemma (that if a prime *p* divides *ab* then it must divide *a* or *b*).

Since each natural number (> 1) has at least one prime factor, and two successive numbers *n* and (*n* + 1) have no factor in common, the product *n*(*n* + 1) has more different prime factors than the number *n* itself. So the chain of pronic numbers:

1×2 = 2 {2}, 2×3 = 6 {2, 3}, 6×7 = 42 {2, 3, 7}, 42×43 = 1806 {2, 3, 7, 43}, 1806×1807 = 3263442 {2, 3, 7, 43, 13, 139}, · · ·

provides a sequence of unlimited growing sets of primes.

### Proof using the irrationality of π

Representing the Leibniz formula for π as an Euler product gives[13]

The numerators of this product are the odd prime numbers, and each denominator is the multiple of four nearest to the numerator.

If there were finitely many primes this formula would show that π is a rational number whose denominator is the product of all multiples of 4 that are one more or less than a prime number, contradicting the fact that π is irrational.

### Proof using information theory

Alexander Shen and others have presented a proof that uses incompressibility:[14]

Suppose there were only *k* primes (*p*_{1}... *p*_{k}). By the fundamental theorem of arithmetic, any positive integer *n* could then be represented as:

where the non-negative integer exponents *e*_{i} together with the finite-sized list of primes are enough to reconstruct the number. Since for all *i*, it follows that all (where denotes the base-2 logarithm).

This yields an encoding for *n* of the following size (using big O notation):

- bits.

This is a much more efficient encoding than representing *n* directly in binary, which takes bits. An established result in lossless data compression states that one cannot generally compress *N* bits of information into less than *N* bits. The representation above violates this by far when *n* is large enough since .

Therefore, the number of primes must not be finite.

## A generalization: Dirichlet's theorem on arithmetic progressions

Dirichlet's theorem states that for any two positive coprime integers *a* and *d*, there are infinitely many primes of the form *a* + *nd*, where *n* is also a positive integer. In other words, there are infinitely many primes that are congruent to *a* modulo *d*. Euclid's theorem is a special case of Dirichlet's theorem for *a = d = 1*. Every case of Dirichlet's theorem yields Euclid's theorem.

## A stronger result: the prime number theorem

Let *π*(*x*) be the prime-counting function that gives the number of primes less than or equal to x, for any real number x. The prime number theorem then states that *x* / log *x* is a good approximation to *π*(*x*), in the sense that the limit of the *quotient* of the two functions *π*(*x*) and *x* / log *x* as x increases without bound is 1:

Using asymptotic notation this result can be restated as

This yields Euclid's theorem, since

## Notes and references

- James Williamson (translator and commentator),
*The Elements of Euclid, With Dissertations*, Clarendon Press, Oxford, 1782, page 63. - Ore, Oystein (1988) [1948],
*Number Theory and its History*, Dover, p. 65 - In general, for any integers
*a*,*b*,*c*if and , then . For more information, see Divisibility. - The exact formulation of Euclid's assertion is: "The prime numbers are more numerous than any proposed multitude of prime numbers".
- Katz, Victor J. (1998),
*A History of Mathematics/ an Introduction*(2nd ed.), Addison Wesley Longman, p. 87 - Michael Hardy and Catherine Woodgold, "Prime Simplicity",
*Mathematical Intelligencer*, volume 31, number 4, fall 2009, pages 44–52. - Franzén, Torkel (2004),
*Inexhaustibility: A Non-exhaustive Treatment*, A K Peters, Ltd, p. 101 - Bostock, Linda; Chandler, Suzanne; Rourke, C. (2014-11-01).
*Further Pure Mathematics*. Nelson Thornes. p. 168. ISBN 9780859501033. - Furstenberg, Harry (1955). "On the infinitude of primes".
*American Mathematical Monthly*.**62**(5): 353. doi:10.2307/2307043. JSTOR 2307043. MR 0068566. - Juan Pablo Pinasco, "New Proofs of Euclid's and Euler's theorems",
*American Mathematical Monthly*, volume 116, number 2, February, 2009, pages 172–173. - Junho Peter Whang, "Another Proof of the Infinitude of the Prime Numbers",
*American Mathematical Monthly*, volume 117, number 2, February 2010, page 181. - Saidak, Filip (December 2006). "A New Proof of Euclid's Theorem".
*American Mathematical Monthly*.**113**(10). doi:10.2307/27642094. - Debnath, Lokenath (2010),
*The Legacy of Leonhard Euler: A Tricentennial Tribute*, World Scientific, p. 214, ISBN 9781848165267. - Shen, Alexander (2016),
*Kolmogorov complexity and algorithmic randomness*(PDF), AMS, p. 245

## External links

- Weisstein, Eric W. "Euclid's Theorem".
*MathWorld*. - Euclid's Elements, Book IX, Prop. 20 (Euclid's proof, on David Joyce's website at Clark University)
- 125 proofs of Euclid's theorem