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 A356482 #14 Oct 14 2023 23:54:27
%S A356482 1,1,16,714,62528,9056720,1960138560,592615689904,238560786221056,
%T A356482 123358665203311104,79683847063011614720
%N A356482 a(n) is the hafnian of a symmetric Toeplitz matrix M(2*n) whose first row consists of 2*n, 2*n-1, ..., 1.
%H A356482 Wikipedia, <a href="https://en.wikipedia.org/wiki/Hafnian">Hafnian</a>
%H A356482 Wikipedia, <a href="https://en.wikipedia.org/wiki/Symmetric_matrix">Symmetric matrix</a>
%H A356482 Wikipedia, <a href="http://en.wikipedia.org/wiki/Toeplitz_matrix">Toeplitz Matrix</a>
%e A356482 a(2) = 16 because the hafnian of
%e A356482 4 3 2 1
%e A356482 3 4 3 2
%e A356482 2 3 4 3
%e A356482 1 2 3 4
%e A356482 equals M_{1,2}*M_{3,4} + M_{1,3}*M_{2,4} + M_{1,4}*M_{2,3} = 16.
%t A356482 k[i_]:=i; M[i_, j_, n_]:=Part[Part[ToeplitzMatrix[Reverse[Array[k, 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 A356482 (PARI) tm(n) = my(m = matrix(n, n, i, j, if (i==1, n-j+1, if (j==1, n-i+1)))); for (i=2, n, for (j=2, n, m[i, j] = m[i-1, j-1]; ); ); m;
%o A356482 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 A356482 Cf. A001792 (determinant of M(n)), A307783.
%Y A356482 Cf. A202038, A336114, A336286, A336400, A338456.
%Y A356482 Cf. A356481, A356483, A356484.
%K A356482 nonn,hard,more
%O A356482 0,3
%A A356482 _Stefano Spezia_, Aug 09 2022
%E A356482 a(6) from _Michel Marcus_, May 02 2023
%E A356482 a(7)-a(10) from _Pontus von Brömssen_, Oct 14 2023