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 A105974 #6 May 26 2017 14:19:48
%S A105974 0,0,1,9,109,2115,65011,3064929,215861689,22255521255,3305532624391,
%T A105974 698140838552949,207432475527926749,85925364190793529675,
%U A105974 49242162128564103516091,38783585510220344176524969
%N A105974 (A001035(n) - 1)/2 where A001035 is the number of partially ordered sets ("posets") with n labeled elements.
%C A105974 Conjectures: a(n) is (pre)periodic modulo m for all m, the lengths of the ending periods being given by 2*A000010(m) if m is a power of 2 and A000010(m) otherwise. A000010 is Euler's totient function.
%Y A105974 Cf. A001035, A000010, A102818.
%K A105974 nonn
%O A105974 0,4
%A A105974 _Gerald McGarvey_, Apr 28 2005