OFFSET
1,2
COMMENTS
This gives the number of different values of A + A' where A runs through the matrices counted in A086215.
PROG
(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) ) ) }
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Max Alekseyev, Dec 13 2005, Nov 09 2006
STATUS
approved