portrait: 71

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. 71 is a centered heptagonal number. It is a Pillai prime, since 9 ! + 1 {\displaystyle 9!+1} is divisible by 71, but 71 is not one more than a multiple of 9.[4] It is part of the last known pair (71, 7) of Brown numbers, since 71 2 = 7 ! + 1 {\displaystyle 71^{2}=7!+1}. 71 is the smallest of thirty-one discriminants of imaginary quadratic fields with class number of 7, negated (see also, Heegner numbers). 71 is the largest number which occurs as a prime factor of an order of a sporadic simple group, the largest (15th) supersingular prime.