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 A130442 #18 Feb 16 2025 08:33:06
%S A130442 4,8,10,20,40,344,4870,6892,17230,68920,250820,296440,317032,368722,
%T A130442 369370,451426,472312,473240,632270,2326472,3186730,3429190,4438760,
%U A130442 4670956,4948456,5509540,8990356,11817604,11841436,13094342,17668360
%N A130442 Even pseudoprimes to base 41.
%H A130442 Amiram Eldar, <a href="/A130442/b130442.txt">Table of n, a(n) for n = 1..366</a> (terms below 10^11; terms 1..146 from Robert G. Wilson v)
%H A130442 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>.
%H A130442 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>.
%t A130442 Do[ f=PowerMod[ 41, 2n-1, 2n ]; If[ f==1, Print[ 2n ] ], {n,2,500000} ]
%t A130442 lst = {}; Do[ If[ PowerMod[41, 2n - 1, 2n] == 1, AppendTo[lst, 2n]], {n, 2, 2^31}]; lst (* _Robert G. Wilson v_, Jun 01 2007 *)
%o A130442 (PARI) is(k) = k > 2 && !(k % 2) && Mod(41, k)^(k-1) == 1; \\ _Amiram Eldar_, Sep 29 2024
%Y A130442 Cf. A020169 (pseudoprimes to base 41).
%Y A130442 Cf. A006935 (even pseudoprimes (or primes) to base 2: n divides 2^n - 2, n even).
%Y A130442 Cf. A130433 (even pseudoprimes to base 3).
%Y A130442 Cf. A090082 (even pseudoprimes to base 5).
%Y A130442 Cf. A090083, A090084, A090085.
%Y A130442 Cf. A130434, A130435, A130436, A130437, A130438, A130439, A130440, A130441, A130443.
%Y A130442 Cf. A020169, A006935, A130433, A090082, A090083, A090084, A090085, A130434, A130435, A130436, A130437, A130438, A130439, A130440, A130441, A130443.
%K A130442 nonn
%O A130442 1,1
%A A130442 _Alexander Adamchuk_, May 26 2007
%E A130442 More terms from _Robert G. Wilson v_, Jun 01 2007