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 A289896 #8 Feb 16 2025 08:33:49
%S A289896 0,0,1,8,45,242,1414,9432,72246,628260,6116319,65856928,776628203,
%T A289896 9950798214,137612550620,2042588039056,32384583304092,546155914771464,
%U A289896 9761655297881037,184309987662192600,3665514979650544153,76586509823743735706,1677182881414143407490
%N A289896 Number of (undirected) cycles in the n-triangular honeycomb rook graph.
%H A289896 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/GraphCycle.html">Graph Cycle</a>
%F A289896 a(n) = sum(k=1..n,A002807(k)).
%F A289896 a(n) ~ exp(1) * (n-1)! / 2. - _Vaclav Kotesovec_, Jun 09 2019
%t A289896 Table[Sum[Binomial[k, l] (l - 1)!, {k, n}, {l, 3, k}]/2, {n, 20}]
%t A289896 Table[Sum[k (2 HypergeometricPFQ[{1, 1, 1 - k}, {2}, -1] - k - 1)/4, {k, n}], {n, 20}]
%Y A289896 Cf. A002807 (cycles in the complete graph).
%K A289896 nonn
%O A289896 1,4
%A A289896 _Eric W. Weisstein_, Jul 14 2017