cp's OEIS Frontend

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.

A085506 Number of {-1,0,1} n X n matrices that have (real and) positive eigenvalues.

Original entry on oeis.org

1, 5, 133, 18905
Offset: 1

Views

Author

Eric W. Weisstein, Jul 02 2003

Keywords

Programs

  • Mathematica
    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];
    Do[Print[n, " ", a[n]], {n, 1, 3}] (* Jean-François Alcover, Dec 09 2018 *)

Extensions

Offset corrected by Max Alekseyev, Oct 18 2008