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A211059 Number of 2 X 2 matrices having all terms in {1,...,n} and positive determinant.

%I A211059 #6 Jan 11 2017 01:31:43
%S A211059 0,5,33,112,288,605,1145,1968,3176,4861,7161,10152,14040,18917,24961,
%T A211059 32352,41312,51949,64585,79320,96472,116277,139025,164840,194184,
%U A211059 227261,264385,305840,352096,403245,459945,522312,590840,665917
%N A211059 Number of 2 X 2 matrices having all terms in {1,...,n} and positive determinant.
%C A211059 For a guide to related sequences, see A210000.
%t A211059 a = 1; b = n; z1 = 35;
%t A211059 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A211059 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A211059 c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
%t A211059 Table[c1[n, n^2] - c[n, 0], {n, 1, z1}]   (* A211059 *)
%t A211059 2*%     (* A211056 *)
%Y A211059 Cf. A210000, A211056.
%K A211059 nonn
%O A211059 1,2
%A A211059 _Clark Kimberling_, Mar 31 2012