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A211061 Number of 2 X 2 matrices having all terms in {1,...,n} and determinant >= n.

%I A211061 #11 Jan 07 2017 02:39:29
%S A211061 0,3,19,69,181,411,785,1419,2334,3674,5478,7994,11093,15249,20347,
%T A211061 26660,34253,43661,54463,67637,82614,100217,120415,143935,169815,
%U A211061 199883,233505,271344,313103,360519,411681,469615,532407,601850,677764
%N A211061 Number of 2 X 2 matrices having all terms in {1,...,n} and determinant >= n.
%C A211061 For a guide to related sequences, see A210000.
%H A211061 Chai Wah Wu, <a href="/A211061/b211061.txt">Table of n, a(n) for n = 1..1000</a>
%t A211061 a = 1; b = n; z1 = 35;
%t A211061 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A211061 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A211061 c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
%t A211061 Table[c1[n, n^2] - c1[n, n - 1], {n, 1, z1}]
%Y A211061 Cf. A210000.
%K A211061 nonn
%O A211061 1,2
%A A211061 _Clark Kimberling_, Mar 31 2012