57 is a prime number

Explanation

It is divisible by 1, 3, 19 and itself, so it is not a prime number. A quick way to check divisibility by 3 is to add all the digits together and see if the result is a multiple of 3: 5 + 7 = 12, which is a multiple of 3 (3 × 4). Fun fact: 57 is nicknamed the "Grothendieck prime" after mathematician Alexander Grothendieck, who reportedly once cited it as an example of a prime.