1,2

M need not be symmetric. For the number of different values of M + M' see A114601. - Max Alekseyev (maxal(AT)cs.ucsd.edu), Dec 13 2005

Eric Weisstein's World of Mathematics, Positive Definite Matrix

(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) ) ) } (Alekseyev)

Cf. A114601, A085656.

Sequence in context: A096348 A015005 A002437 this_sequence A119163 A082160 A109059

Adjacent sequences: A086212 A086213 A086214 this_sequence A086216 A086217 A086218

nonn

Eric Weisstein (eric(AT)weisstein.com), Jul 12, 2003

79505 from Wouter Meeussen (wouter.meeussen(AT)pandora.be), Sep 05 2003

Two more terms from Max Alekseyev (maxal(AT)cs.ucsd.edu), Dec 13 2005

15477341239385927 from Max Alekseyev (maxal(AT)cs.ucsd.edu), Nov 09 2006