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 A085657 #12 Apr 25 2013 03:19:51
%S A085657 1,2,8,61,819,17417,506609,15582436
%N A085657 Number of n X n symmetric positive definite matrices with 2's on the main diagonal and 1's and 0's elsewhere.
%C A085657 Of course the total number of symmetric matrices of this type (not necessarily positive definite) is 2^C(n,2).
%C A085657 This gives the number of different values of A + A' where A runs through the matrices counted in A085656. - _Max Alekseyev_, Dec 13 2005
%H A085657 <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%e A085657 The singular matrix
%e A085657 2 0 1 1
%e A085657 0 2 1 1
%e A085657 1 1 2 0
%e A085657 1 1 0 2
%e A085657 is one of the three 4 X 4 matrices which are not positive definite.
%o A085657 (PARI) { a(n) = M=matrix(n,n,i,j,2*(i==j)); r=0; b(1); r } { b(k) = if(k>n, r++; return); forvec(x=vector(k-1,i,[0,1]), 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 A085657 Cf. A085659, A080858, A083029.
%Y A085657 Cf. A085656, A114601.
%K A085657 nonn
%O A085657 1,2
%A A085657 _N. J. A. Sloane_, Jul 12 2003
%E A085657 More terms from _Max Alekseyev_, Dec 13 2005