How many prime numbers are there?
more than googol, but less the Graham's number
The prime number is a natural number that can be divided, without a remainder, only by itself and by 1. For example, 5 can be divided only by 5 and by 1. There are infinitely many primes, as demonstrated by Euclid around 300 BC. The picture above shows the Ulam spiral, which is a graphical depiction of the set of prime numbers.