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 A130440 #16 Feb 16 2025 08:33:06
%S A130440 6,10,30,66,946,3310,10470,36370,60126,104106,128766,170710,323670,
%T A130440 369370,398266,596926,813430,1145166,1494690,2384866,5960746,6376126,
%U A130440 8178346,18327310,31380922,34102630,37105762,40796526,41950966,41983446
%N A130440 Even pseudoprimes to base 31.
%H A130440 Amiram Eldar, <a href="/A130440/b130440.txt">Table of n, a(n) for n = 1..169</a> (terms below 10^11; terms 1..87 from Robert G. Wilson v)
%H A130440 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>.
%H A130440 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>.
%t A130440 Do[ f=PowerMod[ 31, 2n-1, 2n ]; If[ f==1, Print[ 2n ] ], {n,2,500000} ]
%t A130440 lst = {}; Do[ If[ PowerMod[31, 2n - 1, 2n] == 1, AppendTo[lst, 2n]], {n, 2, 2^31}]; lst (* _Robert G. Wilson v_, Jun 01 2007 *)
%o A130440 (PARI) is(k) = k > 2 && !(k % 2) && Mod(31, k)^(k-1) == 1; \\ _Amiram Eldar_, Sep 29 2024
%Y A130440 Cf. A020159, A006935, A130433, A090082, A090083, A090084, A090085, A130434, A130435, A130436, A130437, A130438, A130439, A130441, A130442, A130443.
%K A130440 nonn
%O A130440 1,1
%A A130440 _Alexander Adamchuk_, May 26 2007
%E A130440 More terms from _Robert G. Wilson v_, Jun 01 2007