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 A356638 #7 Aug 19 2022 09:13:46 %S A356638 3277,29341,49141,80581,88357,104653,196093,314821,458989,476971, %T A356638 489997,800605,838861,873181,877099,1004653,1251949,1302451,1325843, %U A356638 1373653,1397419,1441091,1507963,1509709,1530787,1678541,1811573,1907851,1987021,2004403,2269093 %N A356638 Odd composite numbers k such that 2^((k-1)/2) == -1 (mod k). %C A356638 Counterexamples (pseudoprimes) to the hypothesis that this congruence is sufficient to prove that an odd number is prime. %C A356638 Compare with Proth's theorem. %t A356638 Select[Range[1, 2.5*10^6, 2], CompositeQ[#] && PowerMod[2, (# - 1)/2, #] == # - 1 &] (* _Amiram Eldar_, Aug 19 2022 *) %o A356638 (PARI) forstep(k=3,10^8,2,isprime(k)&&next();Mod(2,k)^((k-1)/2)==-1&&print1(k,", ")) %Y A356638 Cf. A244626, A244628. %K A356638 nonn %O A356638 1,1 %A A356638 _Jeppe Stig Nielsen_, Aug 19 2022