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 A384438 #11 Jun 02 2025 17:44:38 %S A384438 341,1105,1387,1729,1771,2047,2465,2485,2701,2821,3277,3445,4033,4369, %T A384438 4681,5185,5461,6601,7957,8321,8911,9361,10261,10585,11305,11713, %U A384438 11891,13741,13747,13981,14491,15709,15841,16105,16705,18145,18721,19951,23377,28441,29341 %N A384438 Composite numbers k such that ((2^k+1)/3)^k == 1 (mod k^2). %C A384438 If p > 3 is prime, then ((2^p+1)/3)^p == 1 (mod p^2). %C A384438 Fermat pseudoprimes to base 2 not divisible by 3 (A066488) are a proper subsequence. %C A384438 The terms k that are not 2^(k-1) == 1 (mod k) are 1771, 2485, 3445, 5185, 9361, ... %o A384438 (PARI) isok(k) = (k>1) && (k%2) && !isprime(k) && (Mod((2^k+1)/3, k^2)^k == 1); \\ _Michel Marcus_, May 29 2025 %Y A384438 Cf. A001567, A066488 (subsequence), A384148. %K A384438 nonn %O A384438 1,1 %A A384438 _Thomas Ordowski_, May 29 2025 %E A384438 a(18)-a(41) from _Jinyuan Wang_, May 29 2025