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A211147 Number of 2 X 2 matrices having all terms in {-n,...,0,...,n} and nonnegative determinant.

%I A211147 #13 Jan 15 2017 11:47:35
%S A211147 1,57,377,1345,3553,7737,14937,26177,42945,66681,99193,142209,198241,
%T A211147 269049,357593,466433,598401,756281,944057,1164289,1421601,1719161,
%U A211147 2061081,2451329,2895489,3396729,3960569,4591937,5296289,6077881
%N A211147 Number of 2 X 2 matrices having all terms in {-n,...,0,...,n} and nonnegative determinant.
%C A211147 It appears that all terms are of the form 8*k + 1.
%C A211147 For a guide to related sequences, see A210000.
%t A211147 a = -n; b = n; z1 = 30;
%t A211147 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A211147 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A211147 c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
%t A211147 Table[c1[n, 2 n^2], {n, 0, z1}]
%Y A211147 Cf. A210000.
%K A211147 nonn
%O A211147 0,2
%A A211147 _Clark Kimberling_, Apr 03 2012