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A086510 Number of n X n real (0,1)-matrices with all eigenvalues >= 0. 3
1, 2, 13, 261, 15418, 2566333 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..5.

B. D. McKay, F. E. Oggier, G. F. Royle, N. J. A. Sloane, I. M. Wanless and H. S. Wilf, Acyclic digraphs and eigenvalues of (0,1)-matrices, arXiv:math/0310423 [math.CO], 2003.

B. D. McKay, F. E. Oggier, G. F. Royle, N. J. A. Sloane, I. M. Wanless and H. S. Wilf, Acyclic digraphs and eigenvalues of (0,1)-matrices, J. Integer Sequences, 7 (2004), #04.3.3.

Index entries for sequences related to binary matrices

EXAMPLE

a(2)=13 because only 3 of the 16 possible matrices have eigenvalues < 0:

.

  0  1

  1  0

  with eigenvalues {1,-1}

and

  1 1

  1 0

.

  0 1

  1 1

  both with eigenvalues {1.61803..(Golden ratio),-0.61803...}

MATHEMATICA

a[0] = 1; a[n_] := Module[{M, iter, cnt = 0}, M = Table[a[i, j], {i, 1, n}, {j, 1, n}]; iter = Thread[{Flatten[M], 0, 1}]; Do[If[AllTrue[Eigenvalues[ M], NonNegative], cnt++], Evaluate[Sequence @@ iter]]; cnt];

Do[Print[n, " ", a[n]], {n, 0, 5}] (* Jean-Fran├žois Alcover, Dec 09 2018 *)

CROSSREFS

Cf. A003024, A055165, A085656, A098148.

Sequence in context: A276744 A042677 A134296 * A326360 A123113 A126742

Adjacent sequences:  A086507 A086508 A086509 * A086511 A086512 A086513

KEYWORD

nonn,more

AUTHOR

Frederique Oggier (frederique.oggier(AT)epfl.ch) and N. J. A. Sloane, Sep 10 2003

EXTENSIONS

a(5) from Hugo Pfoertner, Sep 26 2017

STATUS

approved

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Last modified December 15 22:02 EST 2019. Contains 330012 sequences. (Running on oeis4.)