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 A019447 #29 Feb 25 2024 09:03:03
%S A019447 1,1,2,4,12,35,120,427,1628,6090,23797,90296,350726,1338076,5165957,
%T A019447 19732508
%N A019447 Number of monomials in expansion of determinant of an n X n Toeplitz matrix [ t(|i-j|) ] in terms of its entries.
%p A019447 with(LinearAlgebra): f:=n->nops([coeffs(Determinant(Matrix(n, (i,j) -> a[abs(i-j)])))]): [seq(f(n),n=0..12)];
%t A019447 f[n_] := Length[ Expand[ Det[ Table[ t[ Abs[ i - j]], {i, 1, n}, {j, 1, n}]]]]; Table[ f[n], {n, 1, 11}]
%Y A019447 Cf. A019448, A086647.
%K A019447 nonn,more,hard
%O A019447 0,3
%A A019447 Herbert S. Wilf
%E A019447 Terms a(12) and a(13) from _Thotsaporn Thanatipanonda_, Jan 26-28 2012
%E A019447 Terms a(14) and a(15) from _Roman Pearce_, Aug 30 2014
%E A019447 a(0)=1 prepended by _Alois P. Heinz_, Feb 25 2024