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.

A prime number has exactly two factors, itself and one. The first ten prime numbers are \({2}\), \({3}\), \({5}\), \({7}\), \({11}\), \({13}\), \({17}\), \({19 ...

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 ...

One of my favorite anecdotes about prime numbers concerns Alexander Grothendieck, who was among the most brilliant mathematicians of the 20th century. According to one account, he was once asked ...