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%I #6 Jan 14 2017 01:43:42
%S 1,13,50,120,244,410,681,981,1431,1948,2623,3315,4373,5323,6588,8009,
%T 9706,11290,13535,15503,18233,20912,23879,26725,30910,34443,38556,
%U 42887,48041,52519,58888,63994,70647,77056,83981,91064,100373
%N Number of 2 X 2 matrices with all elements in {0,1,...,n} and determinant in {0,1,...,n}.
%C See A210000 for a guide to related sequences.
%t a = 0; b = n; z1 = 40;
%t t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t c[n_, k_] := c[n, k] = Count[t[n], k]
%t c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
%t Table[c1[n, n], {n, 0, z1}] (* A209995 *)
%Y Cf. A210000.
%K nonn
%O 0,2
%A _Clark Kimberling_, Mar 18 2012
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