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 A358158 #13 Oct 15 2023 09:26:50
%S A358158 1,0,4,238,31992,9390096,4755878928,3802500283680,4720879431568800,
%T A358158 8379987002639042400,20346893722025317036800
%N A358158 a(n) is the hafnian of the 2n X 2n symmetric matrix defined by M[i,j] = floor(i*j/3).
%C A358158 The matrix M(n) is the n-th principal submatrix of the rectangular array A143974.
%H A358158 Wikipedia, <a href="https://en.wikipedia.org/wiki/Hafnian">Hafnian</a>
%H A358158 Wikipedia, <a href="https://en.wikipedia.org/wiki/Symmetric_matrix">Symmetric matrix</a>
%e A358158 a(2) = 4:
%e A358158 0 0 1 1
%e A358158 0 1 2 2
%e A358158 1 2 3 4
%e A358158 1 2 4 5
%t A358158 M[i_, j_, n_]:=Part[Part[Table[Floor[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 A358158 (PARI) tm(n) = matrix(n, n, i, j, (i*j)\3);
%o A358158 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 A358158 Cf. A143974.
%Y A358158 Cf. A000212 (matrix element M[n,n]), A181286 (trace of M(n)), A358157 (permanent of M(n)).
%K A358158 nonn,hard,more
%O A358158 0,3
%A A358158 _Stefano Spezia_, Nov 01 2022
%E A358158 a(6) from _Michel Marcus_, May 02 2023
%E A358158 a(7)-a(10) from _Pontus von Brömssen_, Oct 15 2023