OFFSET
1,2
COMMENTS
M need not be symmetric. For the number of different values of M + M' see A114601. - Max Alekseyev, Dec 13 2005
LINKS
Eric Weisstein's World of Mathematics, Positive Definite Matrix
MATHEMATICA
Table[Count[Tuples[{-1, 0, 1}, {n, n}], _?PositiveDefiniteMatrixQ], {n, 3}] (* Eric W. Weisstein, Jan 03 2021 *)
PROG
(PARI) { a(n) = M=matrix(n, n, i, j, 2*(i==j)); r=0; b(1); r } { b(k) = local(z, t); if(k>n, z=t=0; for(i=1, n, for(j=1, i-1, if(M[ i, j ]==0, z++); if(abs(M[ i, j ])==1, t++); )); r+=3^z*2^t; return; ); forvec(x=vector(k-1, i, [ -1, 1 ]), for(i=1, k-1, M[ k, i ]=M[ i, k ]=x[ i ]); if( matdet(vecextract(M, 2^k-1, 2^k-1), 1)>0, b(k+1) ) ) } /* Max Alekseyev */
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Eric W. Weisstein, Jul 12 2003
EXTENSIONS
a(4) from Wouter Meeussen, Sep 05 2003
a(5)-a(6) from Max Alekseyev, Dec 13 2005
a(7) from Max Alekseyev, Nov 09 2006
STATUS
approved