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 A239780 #8 Jul 17 2014 22:35:48 %S A239780 1,1,12,279,11128,669605,55937376,6146973259 %N A239780 Number of pairs of functions f, g on a size n set into itself satisfying f(f(g(x))) = f(g(g(x))). %p A239780 s:= proc(n, i) option remember; `if`(i=0, [[]], %p A239780 map(x-> seq([j, x[]], j=1..n), s(n, i-1))) %p A239780 end: %p A239780 a:= proc(n) (l-> add(add(`if`([true$n]=[seq(evalb( %p A239780 f[f[g[i]]]=f[g[g[i]]]), i=1..n)], 1, 0), g=l), f=l))(s(n$2)) %p A239780 end: %p A239780 seq(a(n), n=0..5); # _Alois P. Heinz_, Jul 17 2014 %Y A239780 Cf. A181162, A239769, A239773. %K A239780 nonn,more %O A239780 0,3 %A A239780 _Chad Brewbaker_, Mar 26 2014 %E A239780 a(6)-a(7) from _Giovanni Resta_, Mar 28 2014