Keep going with the other primes (7, 11, 13 etc), removing all the multiples of these as you go. You should end up with a list of 25 prime numbers between 1 and 100. These are shown in the grid below.

“That’s why I’m enthusiastic about this type of prime number.” Whether there are other memorable primes is not yet known. Mathematicians have checked all cases up to n = 60,000; apart from ...

Then list all the numbers that aren’t divisible by those primes. These numbers are the rough primes. In this case, you end up with 50 rough primes: 46 of them are actually prime, while the ...