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 A358162 #16 Oct 15 2023 09:26:59
%S A358162 1,1,11,530,71196,18680148,8825763888,6969574132560,8223753750015600,
%T A358162 14043461354695317600,33726601900489760438400
%N A358162 a(n) is the hafnian of the 2n X 2n symmetric matrix defined by M[i,j] = ceiling(i*j/3).
%C A358162 The matrix M(n) is the n-th principal submatrix of the rectangular array A143977.
%H A358162 Wikipedia, <a href="https://en.wikipedia.org/wiki/Hafnian">Hafnian</a>
%H A358162 Wikipedia, <a href="https://en.wikipedia.org/wiki/Symmetric_matrix">Symmetric matrix</a>
%e A358162 a(2) = 11:
%e A358162 1 1 1 2
%e A358162 1 2 2 3
%e A358162 1 2 3 4
%e A358162 2 3 4 6
%t A358162 M[i_, j_, n_]:=Part[Part[Table[Ceiling[r*c/3], {r, n}, {c, n}], i], j]; a[n_]:=Sum[Product[M[Part[PermutationList[s, 2n], 2i-1], Part[PermutationList[s, 2n], 2i], 2n], {i, n}], {s, SymmetricGroup[2n]//GroupElements}]/(n!*2^n); Array[a, 6, 0]
%o A358162 (PARI) tm(n) = matrix(n, n, i, j, ceil((i*j)/3));
%o A358162 a(n) = my(m = tm(2*n), s=0); forperm([1..2*n], p, s += prod(j=1, n, m[p[2*j-1], p[2*j]]); ); s/(n!*2^n); \\ _Michel Marcus_, May 02 2023
%Y A358162 Cf. A143977.
%Y A358162 Cf. A008810 (matrix element M[n,n]), A070333 (trace of M(n)), A358161 (permanent of M(n)).
%K A358162 nonn,hard,more
%O A358162 0,3
%A A358162 _Stefano Spezia_, Nov 01 2022
%E A358162 a(6) from _Michel Marcus_, May 02 2023
%E A358162 a(7)-a(10) from _Pontus von Brömssen_, Oct 15 2023