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 A215150 #19 Feb 16 2025 08:33:18
%S A215150 13981,18705,23001,55245,63973,72885,75361,107185,126217,129921,
%T A215150 137149,157641,158369,172081,176149,188461,215265,266305,272251,
%U A215150 276013,278545,285541,294409,348161,387731,423793,464185,488881,493697,617093,625921,743665,748657,825265
%N A215150 Pseudoprimes divisible by a smaller pseudoprime.
%C A215150 Here pseudoprime means a Fermat base-2 pseudoprime (a member of A001567).
%C A215150 Pseudoprimes by which the numbers from sequence are divisible: 341, 645, 561, 1905, 1729, 645, 341, 1105, 1387 and 1729, 341, 2047, 561, 5461, 2821, 1387, 1729, 1905, 1105, 2047, 1387, 2465 and 3277, 4681, 2701 and 4033 and 7957, 341, 5461, 4369, 5461, 2701, 4369, 5461, 10261, 1105, 1729, 1387 and 11305.
%C A215150 A pseudoprime can be divisible by more than one pseudoprime: e.g. 126217, 278545, 294409, 825265.
%H A215150 Arkadiusz Wesolowski, <a href="/A215150/b215150.txt">Table of n, a(n) for n = 1..1000</a>
%H A215150 G. L. Honaker, Jr. and Chris Caldwell, <a href="https://t5k.org/curios/cpage/25271.html">Prime Curios! 13981</a>
%H A215150 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PouletNumber.html">Poulet Number</a>
%H A215150 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>
%e A215150 Since 2^13980 = 1 mod 13981 and 13981 = 11 * 31 * 41, 13981 is a pseudoprime, and it is divisible by 341, a smaller pseudoprime. 13981 is therefore in the sequence.
%e A215150 The pseudoprimes 75361, 129921, 348161, etc., are also divisible by 341.
%t A215150 lst1 = {}; lst2 = {}; r = 10^6; Do[If[! PrimeQ[n] && PowerMod[2, n - 1, n] == 1, AppendTo[lst1, n]], {n, 1, r, 2}]; l = Length[lst1]; Do[p = lst1[[a]]; b = 1; While[True, t = lst1[[b]]; If[p < 3*t, Break[]]; If[Divisible[p, t], AppendTo[lst2, p]; Break[]]; b++], {a, 2, l}]; lst2
%Y A215150 Cf. A001567, A214305.
%K A215150 nonn
%O A215150 1,1
%A A215150 _Arkadiusz Wesolowski_, Aug 04 2012