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A211148 Number of 2 X 2 matrices having all terms in {-n,...,0,..,n} and positive determinant.

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