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 A337846 #13 Sep 26 2020 11:29:34 %S A337846 17,257,457,1297,6481,11953,26321,47521,47881,49681,65537,74449, %T A337846 157081,165601,278497,333433,476737,557041,560737,576721,1033057, %U A337846 1266841,1329337,1463617,1468897,2291041,2422201,2754481,2851633,2969137,3255281 %N A337846 Odd integers k such that 2^((k-1)/2) == 1 (mod k*(k-2)). %C A337846 Computed terms are prime. Is this a possible primality test or are there pseudo primes? Terms are of the form 8k+1. %C A337846 The Fermat number F(5) = A000215(5) = 4294967297 = 641*6700417 is the smallest composite counterexample. - _Hugo Pfoertner_, Sep 26 2020 %t A337846 Select[Range[3, 10^6, 2], PowerMod[2, (# - 1)/2, #*(# - 2)] == 1 &] (* _Amiram Eldar_, Sep 26 2020 *) %o A337846 (PARI) is(n) = n%2 && n>=3 && Mod(2, n*(n-2))^((n-1)/2) == 1 %Y A337846 Cf. A081762, A337818. %K A337846 nonn %O A337846 1,1 %A A337846 _Benoit Cloitre_, Sep 26 2020