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 A125587 Call an n X n matrix robust if the top left i X i submatrix is invertible for all i = 1..n. Sequence gives number of n X n robust real {0,1}-matrices. 8
 1, 4, 68, 5008, 1603232, 2224232640 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS An upper bound is the total number of {0,1}-matrices, 2^(n^2). Comment from Michael Kleber, Jan 05 2006: A lower bound is 2^(n^2-n), A053763. For given the principal n-1 X n-1 submatrix A, the 2n-2 further entries (excluding the bottom right corner) can be filled in arbitrarily and then there is always at least one choice for the last entry which makes the matrix invertible. Comment from N. J. A. Sloane, Jan 06 2006: Let the matrix be [A b; c d], where A is n-1 X n-1, b is n-1 X 1, c is 1 X n-1, d is 0 or 1. The matrix is singular iff d = c A^(-1) b, which for given A, b, c has at most one solution d. Suppose A = identity, as in A125586. Then if d=0 there are 3^(n-1) choices for b and c, while if d=1 there are (n-1)*3^(n-2) choices for b and c. This proves the formula in A125586. LINKS Table of n, a(n) for n=1..6. EXAMPLE a(2) = 4 from: 10 10 11 11 01 11 01 10 CROSSREFS Cf. A125586, A126603, A125593. Sequence in context: A012484 A174809 A363427 * A134794 A248027 A093852 Adjacent sequences: A125584 A125585 A125586 * A125588 A125589 A125590 KEYWORD nonn,more AUTHOR N. J. A. Sloane and Vinay Vaishampayan, Jan 05 2007 EXTENSIONS a(5) and a(6) from Brendan McKay, Jan 06 2007 STATUS approved

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Last modified December 9 06:02 EST 2023. Contains 367685 sequences. (Running on oeis4.)