Skip to content

Why primes above 3 give p squared minus 1 divisible by 24

Question

Let pp be any prime larger than 33. Explain why 2424 always divides p2−1p^2-1.

Short answer

Factor p2−1=(p−1)(p+1)p^2-1=(p-1)(p+1). Since pp is odd, p−1p-1 and p+1p+1 are consecutive even numbers, one of them a multiple of 44, so 88 divides the product. Among p−1,p,p+1p-1,p,p+1 one is a multiple of 33 and it is not pp, so 33 divides the product too; hence 2424 does.

Adapted from Crack, Heard on the Street, Q1.49, p.19; question and answer rewritten by SelfMadeQuant.com.

See the hint and the full worked solution in the question bank (free account).

Open the worked solution