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 A086256 #14 Jul 22 2025 13:11:03
%S A086256 0,0,0,0,0,0,0,0,0,1,1,0,0,1,1,1,0,1,0,2,1,4,1,2,1,1,0,13,4,5,0,2,2,1,
%T A086256 1,13,1,1,4,7,1,11,4,14,9,4,4,28,0,12,11,12,4,2,5,28,4,26,1,63,0,1,5,
%U A086256 12,1,29,1,12,2,44,4,101,4,11,27,12,1,26,4,15,4,11,1,75,1,11,14,36,0,40,11
%N A086256 Number of base-2 Fermat pseudoprimes that divide 2^n-1.
%C A086256 A base-2 Fermat pseudoprime is a composite number x such that 2^x = 2 mod x.
%H A086256 Amiram Eldar, <a href="/A086256/b086256.txt">Table of n, a(n) for n = 1..419</a>
%H A086256 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Pseudoprime.html">Pseudoprime</a>
%F A086256 a(n) = Sum{d|n} A086249(d), the Mobius transform of A086249.
%t A086256 Table[d=Divisors[2^n-1]; cnt=0; Do[m=d[[i]]; If[ !PrimeQ[m]&&PowerMod[2, m, m]==2, cnt++ ], {i, Length[d]}]; cnt, {n, 100}]
%Y A086256 Cf. A001567 (base-2 pseudoprimes), A046801, A086249.
%K A086256 nonn,hard
%O A086256 1,20
%A A086256 _T. D. Noe_, Jul 14 2003