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 A239752 #32 Dec 25 2023 17:40:59
%S A239752 1,1,6,123,4792,294645,26351856,3213829339,511432765824,
%T A239752 102813166760265,25450790212460800,7599894406225438911,
%U A239752 2691706949197641133056,1114869818722491048119773,533573397145124307890731008,292063395009538745067415219875,181221082305680372426427865071616
%N A239752 Number of pairs of functions (f,g) from a set of n elements into itself satisfying f(f(x)) = g(f(f(x))).
%C A239752 Also pairs of functions where f(f(f(x))) = g(f(f(x))).
%H A239752 Alois P. Heinz, <a href="/A239752/b239752.txt">Table of n, a(n) for n = 0..160</a>
%F A239752 a(n) = n! Sum_{d = 0..n} n^(n-d) Sum_{c = 0..(n-d)/2} S2(c+d,d) Sum_{b = c..n-c-d} S2(b,c)/(b! (n-b-c-d)!) d^(n-b-c-d). - _David Einstein_, Oct 23 2016
%t A239752 t1[n_, d_, c_] := StirlingS2[c + d, d] Sum[ StirlingS2[b, c]/(b! (n - b - c - d)!) d^(n - b - c - d), {b, c, n - c - d}]
%t A239752 a[n_] := If[n == 0, 1, n! Sum[n^(n - d) Sum[t1[n, d, c], {c, 0, Floor[(n - d)/2]}], {d, 1, n}]]
%t A239752 Table[a[n], {n, 0, 20}] (* _David Einstein_, Oct 23 2016 *)
%Y A239752 Cf. A000248, A000949.
%K A239752 nonn
%O A239752 0,3
%A A239752 _Chad Brewbaker_, Mar 26 2014
%E A239752 a(6)-a(7) from _Giovanni Resta_, Mar 28 2014
%E A239752 a(8)-a(10) from _Paul Boddington_, Feb 23 2015
%E A239752 a(11)-a(16) from _David Einstein_, Oct 23 2016