71 (number)

71 (seventy-one) is the natural number following 70 and preceding 72.

In mathematics
71 is the 20th prime number. Because both rearrangements of its digits (17 and 71) are prime numbers, 71 is an emirp and more generally a permutable prime. It is the largest number which occurs as a prime factor of an order of a sporadic simple group, the largest (15th) supersingular prime.

It is a Pillai prime, since $$9!+1$$ is divisible by 71, but 71 is not one more than a multiple of 9. It is part of the last known pair (71, 7) of Brown numbers, since $$71^{2}=7!+1$$.

It is centered heptagonal number.