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%I #6 Mar 30 2012 16:49:44
%S 1,2,8,45,338,3178,34346,396659,4694705
%N Number of n X n symmetric positive semi-definite matrices with 2's on the main diagonal and -1's and 0's elsewhere.
%C Of course the total number of symmetric matrices of this type (not necessarily positive definite) is 2^C(n,2).
%C This gives the number of different values of A + A' where A runs through the matrices counted in A127503.
%H <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%e The 5 X 5 matrix
%e 2 -1 -1 -1 -1
%e -1 2 -1 -1 -1
%e -1 -1 2 -1 -1
%e -1 -1 -1 2 -1
%e -1 -1 -1 -1 2
%e with eigenvalues -2, 3, 3, 3, 3 is an example of one which is not positive semi-definite.
%Y Cf. A084552, A085659, A080858, A083029.
%K nonn
%O 1,2
%A _N. J. A. Sloane_, Jul 13 2003
%E a(6)-a(9) from Max Alekseyev, Jan 16 2006
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