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 A360878 #11 May 28 2025 10:51:43
%S A360878 1,12,129,1984,45945,1524156,68838217,4070403744,305642504529,
%T A360878 28440008182540,3214141725643761,433856895597946272,
%U A360878 68964321078341276809,12753724616472980432124,2715405762438952565521785,659549661987730244294458816,181293528280954206831103494177
%N A360878 Number of (undirected) paths in the 2 X n rook graph.
%H A360878 Andrew Howroyd, <a href="/A360878/b360878.txt">Table of n, a(n) for n = 1..100</a>
%F A360878 a(n) = (Sum_{k=2..n} binomial(n,k)*k!) + (Sum_{k=1..n} k*binomial(n,k)*binomial(k-1, floor(k/2)) * (Sum_{i=0..n-k} binomial(n-k,i)*(floor(k/2)+i)!) * (Sum_{i=0..n-k} binomial(n-k,i)*(floor((k-1)/2)+i)!)). - _Andrew Howroyd_, May 28 2025
%o A360878 (PARI) a(n)={sum(k=2, n, binomial(n,k)*k!) + sum(k=1, n, k*binomial(n,k)*binomial(k-1,k\2)*sum(i=0, n-k, binomial(n-k,i)*(k\2+i)!)*sum(i=0, n-k, binomial(n-k,i)*((k-1)\2+i)!))} \\ _Andrew Howroyd_, May 28 2025
%Y A360878 Row 2 of A360877.
%Y A360878 Cf. A096121, A276356, A341500.
%K A360878 nonn
%O A360878 1,2
%A A360878 _Andrew Howroyd_, Feb 25 2023
%E A360878 a(8) onwards from _Andrew Howroyd_, May 28 2025