1,2

Of course the total number of symmetric matrices of this type (not necessarily positive definite) is 2^C(n,2).

This gives the number of different values of M + M' where M runs through the matrices counted in A127502.

Table of n, a(n) for n=1..10.

Index entries for sequences related to binary matrices

The singular matrix

2 -1 -1

-1 2 -1

-1 -1 2

is the only 3 X 3 matrix of this type which is not positive definite.

Cf. A084553, A085659, A080858, A083029.

Sequence in context: A114160 A145159 A317985 * A094664 A001858 A233335

Adjacent sequences: A084549 A084550 A084551 * A084553 A084554 A084555

nonn

N. J. A. Sloane, Jul 13 2003

a(6)-a(10) from Max Alekseyev, Jan 16 2006

approved