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 A209991 #10 Oct 23 2018 16:15:53
%S A209991 1,13,38,79,136,209,302,407,536,681,846,1015,1240,1441,1678,1951,2240,
%T A209991 2505,2854,3151,3552,3945,4326,4687,5216,5657,6110,6615,7192,7649,
%U A209991 8342,8831,9472,10105,10702,11407,12272,12857,13526,14279,15224
%N A209991 Number of 2 X 2 matrices with all elements in {0,1,...,n} and determinant in {0,1}.
%C A209991 See A210000 for a guide to related sequences.
%F A209991 A059306 + A171503.
%t A209991 a = 0; b = n; z1 = 40;
%t A209991 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A209991 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A209991 c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, 1}]
%t A209991 Table[c1[n, 1], {n, 0, z1}] (* A209991 *)
%Y A209991 Cf. A059306, A171503, A210000.
%K A209991 nonn
%O A209991 0,2
%A A209991 _Clark Kimberling_, Mar 18 2012