1,2

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 (njas(AT)research.att.com), 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.

a(2) = 4 from:

10 10 11 11

01 11 01 10

Cf. A125586, A126603, A125593.

Sequence in context: A009520 A009791 A012484 this_sequence A134794 A093852 A065573

Adjacent sequences: A125584 A125585 A125586 this_sequence A125588 A125589 A125590

nonn,more

N. J. A. Sloane (njas(AT)research.att.com) and Vinay Vaishampayan (vinay(AT)research.att.com), Jan 05 2007

a(5) and a(6) from Brendan McKay (bdm(AT)cs.anu.edu.au), Jan 06 2007