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 A005535 M5081 #20 Jul 20 2022 07:26:08
%S A005535 1,19,916,91212,12888450,2411213698,575737451509,171049953499862,
%T A005535 61944438230597774,26879022100485977540,13773587720396658214925,
%U A005535 8231894671550187551622795,5676740663627528580559535893,4474748487205893704072253926113
%N A005535 Number of semi-regular digraphs (with loops) on n unlabeled nodes with each node having out-degree 3.
%D A005535 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A005535 Andrew Howroyd, <a href="/A005535/b005535.txt">Table of n, a(n) for n = 3..50</a>
%H A005535 S. A. Choudum and K. R. Parthasarathy, <a href="http://dx.doi.org/10.1016/1385-7258(72)90047-9">Semi-regular relations and digraphs</a>, Nederl. Akad. Wetensch. Proc. Ser. A. {75}=Indag. Math. 34 (1972), 326-334.
%t A005535 permcount[v_] := Module[{m = 1, s = 0, k = 0, t}, For[i = 1, i <= Length[v], i++, t = v[[i]]; k = If[i > 1 && t == v[[i - 1]], k + 1, 1]; m *= t*k; s += t]; s!/m];
%t A005535 edges[v_, k_] := Product[SeriesCoefficient[Product[g = GCD[v[[i]], v[[j]]]; (1 + x^(v[[j]]/g) + O[x]^(k + 1))^g, {j, 1, Length[v]}], {x, 0, k}], {i, 1, Length[v]}];
%t A005535 a[n_] := Module[{s = 0}, Do[s += permcount[p]*edges[p, 3], {p, IntegerPartitions[n]}]; s/n!];
%t A005535 Table[a[n], {n, 3, 20}] (* _Jean-François Alcover_, Jul 20 2022, after _Andrew Howroyd_ in A259471 *)
%Y A005535 Column k=3 of A259471.
%Y A005535 Cf. A003286, A185193.
%K A005535 nonn,nice
%O A005535 3,2
%A A005535 _N. J. A. Sloane_
%E A005535 a(7) from _Sean A. Irvine_, Jul 07 2016
%E A005535 Terms a(8) and beyond from _Andrew Howroyd_, Sep 13 2020