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 A087488 Number of n X n (-1,1)-matrices with all eigenvalues >= 0. 0
 1, 1, 6, 64, 4744 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS 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. 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 EXAMPLE For n = 2 the six matrices are (+ means +1, - means -1): ++ +- -- -+ +- ++ -- +- ++ -+ -+ ++ with eigenvalues 00 00 00 00 20 20 respectively. CROSSREFS Cf. A086510, A003024, A055165, A085656. Sequence in context: A156887 A173500 A141008 * A179879 A099343 A189509 Adjacent sequences:  A087485 A087486 A087487 * A087489 A087490 A087491 KEYWORD nonn,more AUTHOR Frederique Oggier (frederique.oggier(AT)epfl.ch) and N. J. A. Sloane, Oct 24 2003 STATUS approved

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