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 A176817 #11 Jun 27 2024 08:43:03 %S A176817 1,4,8,16,28,32,64,112,128,256,341,448,496,512,561,645,1016,1024,1105, %T A176817 1288,1387,1729,1792,1905,2044,2047,2048,2416,2465,2701,2821,3277, %U A176817 4033,4096,4369,4371,4672,4681,4984,5461,6601,7168,7936,7957,8128,8192,8321,8481,8911 %N A176817 Nonprimes k such that 2^(k-1) == 4^(k-1) (mod k). %C A176817 All powers of two are present. %t A176817 fQ[n_] := !PrimeQ@ n && PowerMod[2, n - 1, n] == PowerMod[4, n - 1, n]; Select[ Range@ 10000, fQ] %Y A176817 Cf. A062173, A062175, A176176. %K A176817 nonn %O A176817 1,2 %A A176817 _Juri-Stepan Gerasimov_, Dec 07 2010