This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A086215 #20 Feb 16 2025 08:32:50
%S A086215 1,7,311,79505,105311665,642005451319,15477341239385927
%N A086215 Number of (-1,0,1) n X n matrices M that are positive definite.
%C A086215 M need not be symmetric. For the number of different values of M + M' see A114601. - _Max Alekseyev_, Dec 13 2005
%H A086215 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PositiveDefiniteMatrix.html">Positive Definite Matrix</a>
%t A086215 Table[Count[Tuples[{-1, 0, 1}, {n, n}], _?PositiveDefiniteMatrixQ], {n, 3}] (* _Eric W. Weisstein_, Jan 03 2021 *)
%o A086215 (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_ */
%Y A086215 Cf. A114601, A085656.
%K A086215 nonn,hard,more
%O A086215 1,2
%A A086215 _Eric W. Weisstein_, Jul 12 2003
%E A086215 a(4) from _Wouter Meeussen_, Sep 05 2003
%E A086215 a(5)-a(6) from _Max Alekseyev_, Dec 13 2005
%E A086215 a(7) from _Max Alekseyev_, Nov 09 2006