This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A273786 #14 Dec 07 2020 01:48:27 %S A273786 5,7,8,9,10,13,17,18,19,21,22,23,24,25,26,27,28,29,30,31,32,33,35,37, %T A273786 38,40,41,42,43,44,45,46,48,49,50,51,53,54,55,57,60,61,62,63,64,65,67, %U A273786 68,69,70,71,73,74,75,76,77,78,79,80,81,82,85,89,91,93,94 %N A273786 Numbers n where a prime p < n exists such that n^(p-1) == 1 (mod p^2), i.e., such that p is a base-n Wieferich prime. %C A273786 Numbers n such that A255920(n) > 0. %C A273786 Complement of A255921. - _Felix Fröhlich_, Dec 03 2020 %H A273786 Felix Fröhlich, <a href="/A273786/b273786.txt">Table of n, a(n) for n = 1..10000</a> %e A273786 The prime 5 satisfies 24^(5-1) == 1 (mod 5^2) and 5 < 24, so 24 is a term of the sequence. %t A273786 Select[Range@ 94, Function[n, Count[Prime@ Range@ PrimePi@ n, p_ /; Mod[n^(p - 1), p^2] == 1] > 0]] (* _Michael De Vlieger_, May 30 2016 *) %o A273786 (PARI) is(n) = forprime(p=1, n-1, if(Mod(n, p^2)^(p-1)==1, return(1))); 0 %Y A273786 Cf. A255920, A255921, A273785. %K A273786 nonn %O A273786 1,1 %A A273786 _Felix Fröhlich_, May 30 2016