Euclid's Theorems

A theorem sometimes called "Euclid's first theorem" or Euclid's principle states that if is a prime and , then or (where means divides). A corollary is that (Conway and Guy 1996). The fundamental theorem of arithmetic is another corollary (Hardy and Wright 1979).

Euclid's second theorem states that the number of primes is infinite. This theorem, also called the infinitude of primes theorem, was proved by Euclid in Proposition IX.20 of the Elements (Tietze 1965, pp. 7-9). Ribenboim (1989) gives nine (and a half) proofs of this theorem. Euclid's elegant proof proceeds as follows. Given a finite sequence of consecutive primes 2, 3, 5, ..., , the number

(1)

known as the th Euclid number when is the th prime, is either a new prime or the product of primes. If is a prime, then it must be greater than the previous primes, since one plus the product of primes must be greater than each prime composing the product. Now, if is a product of primes, then at least one of the primes must be greater than . This can be shown as follows.

If is composite and has no prime factors greater than , then one of its factors (say ) must be one of the primes in the sequence, 2, 3, 5, ..., . It therefore divides the product . However, since it is a factor of , it also divides . But a number which divides two numbers and also divides their difference , so must also divide

(2)

However, in order to divide 1, must be 1, which is contrary to the assumption that it is a prime in the sequence 2, 3, 5, .... It therefore follows that if is composite, it has at least one factor greater than . Since is either a prime greater than or contains a prime factor greater than , a prime larger than the largest in the finite sequence can always be found, so there are an infinite number of primes. Hardy (1992) remarks that this proof is "as fresh and significant as when it was discovered" so that "two thousand years have not written a wrinkle" on it.

A similar argument shows that must be either prime or be divisible by a prime . Kummer used a variation of this proof, which is also a proof by contradiction. It assumes that there exist only a finite number of primes , , ..., . Now define and consider . It must be a product of primes, so it has a prime divisor in common with . Therefore, which is nonsense, so we have proved the initial assumption is wrong by contradiction.

It is also true that there are runs of composite numbers which are arbitrarily long. This can be seen by defining

(3)

where is a factorial. Then the consecutive numbers , , ..., are composite, since

			(4)
			(5)
			(6)
			(7)
			(8)
			(9)

Guy (1981, 1988) points out that while is not necessarily prime, letting be the next prime after , the number is conjectured always to be a prime known as a Fortunate prime, though this has not been proven.