 For example, in the case of two events, a number is divisible by primes "p" and "q" if and only if it is divisible by "pq"; the latter event has probability 1/"pq".If one makes the heuristic assumption that such reasoning can be extended to infinitely many divisibility events, one is led to guess that the probability that two numbers are coprime is given by a product over all primes,

Here "ζ" refers to the Riemann zeta function, the identity relating the product over primes to "ζ"(2) is an example of an Euler product, and the evaluation of "ζ"(2) as "π"2/6 is the Basel problem, solved by Leonhard Euler in 1735.There is no way to choose a positive integer at random so that each positive integer occurs with equal probability, but statements about "randomly chosen integers" such as the ones above can be formalized by using the notion of "natural density".