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A114601 Number of n X n symmetric positive definite matrices with 2's on the main diagonal and -1, 0, or 1 elsewhere. 2

%I

%S 1,3,23,393,13089,737595,58969079

%N Number of n X n symmetric positive definite matrices with 2's on the main diagonal and -1, 0, or 1 elsewhere.

%C This gives the number of different values of A + A' where A runs through the matrices counted in A086215.

%o (PARI) { a(n) = M=matrix(n,n,i,j,2*(i==j)); r=0; m=n; b(1); r } { 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) ) ) }

%Y Cf. A086215, A085657.

%K hard,nonn,more

%O 1,2

%A _Max Alekseyev_, Dec 13 2005, Nov 09 2006

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Last modified August 9 04:39 EDT 2020. Contains 336319 sequences. (Running on oeis4.)