

A085658


Number of n X n symmetric positive semidefinite matrices with 2's on the main diagonal and 1's and 0's elsewhere.


6




OFFSET

1,2


COMMENTS

Of course the total number of symmetric matrices of this type (not necessarily positive semidefinite) is 2^C(n,2).
This gives the number of different values of M + M' where M runs through the matrices counted in A038379.  Max Alekseyev, Nov 11 2006


LINKS

Table of n, a(n) for n=1..8.
Index entries for sequences related to binary matrices


EXAMPLE

The matrix
2 0 0 0 1
0 2 0 1 1
0 0 2 1 1
0 1 1 2 0
1 1 1 0 2
is one of the 100 5 X 5 matrices which are not positive semidefinite.
Its eigenvalues are approximately [2., 0.135779205069857, 4.135779205069857, 1.337846553138044, 2.662153446861956]


CROSSREFS

Cf. A085658, A080858, A083029.
Sequence in context: A134956 A011803 A007625 * A153541 A153569 A153532
Adjacent sequences: A085655 A085656 A085657 * A085659 A085660 A085661


KEYWORD

nonn,more


AUTHOR

N. J. A. Sloane, Jul 12 2003


EXTENSIONS

3 more terms from Max Alekseyev, Nov 08 2006


STATUS

approved



