A114601 Number of n X n symmetric positive definite matrices with 2's on the main diagonal and -1, 0, or 1 elsewhere.
1, 3, 23, 393, 13089, 737595, 58969079
Offset: 1
Programs
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PARI
{ b(k) = if(k>m, r++; 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) ) ) } { a(n) = local(M,r,m); M=matrix(n,n,i,j,2*(i==j)); r=0; m=n; b(1); r }
Comments