

A084553


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


2




OFFSET

1,2


COMMENTS

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 A + A' where A runs through the matrices counted in A127503.


LINKS

Table of n, a(n) for n=1..9.
Index entries for sequences related to binary matrices


EXAMPLE

The 5 X 5 matrix
2 1 1 1 1
1 2 1 1 1
1 1 2 1 1
1 1 1 2 1
1 1 1 1 2
with eigenvalues 2, 3, 3, 3, 3 is an example of one which is not positive semidefinite.


CROSSREFS

Cf. A084552, A085659, A080858, A083029.
Sequence in context: A290445 A152401 A009345 * A144164 A003091 A119501
Adjacent sequences: A084550 A084551 A084552 * A084554 A084555 A084556


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Jul 13 2003


EXTENSIONS

a(6)a(9) from Max Alekseyev, Jan 16 2006


STATUS

approved



