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 A115225 #3 Mar 30 2012 17:22:41
%S A115225 0,28,468,2688,12400,37260,100548,215040,454896,883900,1609300,
%T A115225 2618496,4453488,6913900,10548900,15138816,22713088,31256604,44563284,
%U A115225 59459200,81302760,107538508,141587908,178808832,232500000,295476220,368465760
%N A115225 Number of 3 X 3 symmetric matrices over Z(n) having nonzero determinant.
%F A115225 a(n)=n^6-A115223(n). For prime n, a(n)=(n^3-1)(n-1)n^2.
%t A115225 Table[cnt=0; Do[m={{a, b, c}, {b, d, e}, {c, e, f}}; If[Det[m, Modulus->n]>0, cnt++ ], {a, 0, n-1}, {b, 0, n-1}, {c, 0, n-1}, {d, 0, n-1}, {e, 0, n-1}, {f, 0, n-1}]; cnt, {n, 2, 20}]
%Y A115225 Cf. A115222 (number of 3 X 3 matrices over Z(n) having nonzero determinant).
%K A115225 nonn
%O A115225 1,2
%A A115225 _T. D. Noe_, Jan 16 2006