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%I #6 Jan 11 2017 01:31:26
%S 24,248,1056,3008,6904,13624,24448,40576,63640,95288,137632,192384,
%T 262392,349688,457088,587520,744344,930104,1149152,1404160,1699640,
%U 2039544,2428352,2869312,3368472,3929912,4558688,5259712,6039480
%N Number of 2 X 2 matrices having all terms in {-n,...,0,..,n} and positive determinant.
%C For a guide to related sequences, see A210000.
%t a = -n; b = n; z1 = 30;
%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 Table[c[n, 0], {n, 1, z1}] (* A209981 *)
%t c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 1, m}]
%t t = Table[c1[n, 2*n^2], {n, 1, z1}] (* A211148 *)
%t 2 t (* A211149 *)
%t t/8 (* integers *)
%Y Cf. A210000.
%K nonn
%O 1,1
%A _Clark Kimberling_, Apr 04 2012
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