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 A090083 #16 Sep 19 2024 01:50:00
%S A090083 4,8,28,52,286,364,532,616,946,1036,1288,2806,2926,3052,4376,4636,
%T A090083 5356,6364,8744,8866,11476,12124,15964,17446,19096,19684,21196,21736,
%U A090083 24046,24388,26596,31876
%N A090083 Even pseudoprimes to base 9.
%H A090083 Amiram Eldar, <a href="/A090083/b090083.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Charles R Greathouse IV)
%H A090083 <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>.
%t A090083 Do[s=Mod[ -1+9^(n-1), n]; If[Equal[s, 0]&&!PrimeQ[n]&&EvenQ[n], Print[n]], {n, 1, 1000000}]
%o A090083 (PARI) is(n)=Mod(9, n)^(n-1)==1&&!isprime(n)&&n%2==0 \\ _Charles R Greathouse IV_, Apr 12 2012
%o A090083 (PARI) p=2; forprime(q=3, 1e8, forstep(n=p+1, q-1, 2, if(Mod(9, n)^(n-1)==1, print1(n", "))); p=q) \\ _Charles R Greathouse IV_, Apr 12 2012
%Y A090083 Cf. A020138.
%Y A090083 Cf. A006935, A130433, A090082, A090084, A090085, A130434, A130435, A130436, A130438, A130439, A130440, A130441, A130442, A130443.
%K A090083 nonn
%O A090083 1,1
%A A090083 _Labos Elemer_, Nov 25 2003