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 A085506 #20 Feb 16 2025 08:32:50
%S A085506 1,5,133,18905
%N A085506 Number of {-1,0,1} n X n matrices that have (real and) positive eigenvalues.
%H A085506 B. D. McKay, F. E. Oggier, G. F. Royle, N. J. A. Sloane, I. M. Wanless and H. S. Wilf, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL7/Sloane/sloane15.html">Acyclic digraphs and eigenvalues of (0,1)-matrices</a>, J. Integer Sequences, 7 (2004), #04.3.3.
%H A085506 B. D. McKay, F. E. Oggier, G. F. Royle, N. J. A. Sloane, I. M. Wanless and H. S. Wilf, <a href="http://arXiv.org/abs/math.CO/0310423">Acyclic digraphs and eigenvalues of (0,1)-matrices</a>, arXiv:math/0310423 [math.CO], 2003.
%H A085506 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PositiveDefiniteMatrix.html">Positive Definite Matrix</a>
%H A085506 <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%t A085506 a[n_] := Module[{M, iter, cnt = 0}, M = Table[a[i, j], {i, 1, n}, {j, 1, n}]; iter = Thread[{Flatten[M], -1, 1}]; Do[If[AllTrue[Eigenvalues[M], If[Im[#] != 0, False, Positive[#]]&], cnt++], Evaluate[Sequence @@ iter]]; cnt];
%t A085506 Do[Print[n, " ", a[n]], {n, 1, 3}] (* _Jean-François Alcover_, Dec 09 2018 *)
%K A085506 nonn,more
%O A085506 1,2
%A A085506 _Eric W. Weisstein_, Jul 02 2003
%E A085506 Offset corrected by _Max Alekseyev_, Oct 18 2008