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 A115077 #10 Oct 31 2023 11:14:49
%S A115077 0,4,18,44,100,180,294,432,630,900,1210,1548,2028,2548,3150,3744,4624,
%T A115077 5436,6498,7500,8820,10164,11638,13104,14900,16900,18792,20972,23548,
%U A115077 26100,28830,31360,34848,38148,41650,44676,49284,53428,57798,62000
%N A115077 Number of 2 X 2 symmetric matrices over Z(n) having nonzero determinant.
%F A115077 a(n) = n^3 - A115075(n).
%F A115077 For squarefree n, a(n) = (n-1)*n^2.
%t A115077 Table[cnt=0; Do[m={{a, b}, {b, c}}; If[Det[m, Modulus->n]>0, cnt++ ], {a, 0, n-1}, {b, 0, n-1}, {c, 0, n-1}]; cnt, {n, 50}]
%t A115077 f[p_, e_] := p^e*(p^e + p^(e-1) - p^(Ceiling[e/2] - 1)); a[1] = 0; a[n_] := n^3 - Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* _Amiram Eldar_, Oct 31 2023 *)
%o A115077 (PARI) a(n) = {my(f = factor(n), p, e); n^3 - prod(i = 1, #f~, p = f[i, 1]; e = f[i, 2]; p^e*(p^e + p^(e-1) - p^((e+1)\2 - 1)));} \\ _Amiram Eldar_, Oct 31 2023
%Y A115077 Cf. A005353 (number of 2 X 2 matrices over Z(n) having nonzero determinant), A115075.
%K A115077 nonn,easy
%O A115077 1,2
%A A115077 _T. D. Noe_, Jan 12 2006