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A211062 Number of 2 X 2 matrices having all terms in {1,...,n} and determinant >= 2n.

%I A211062 #11 Jan 07 2017 02:39:32
%S A211062 0,0,5,29,93,235,493,936,1622,2642,4057,6055,8608,12001,16297,21619,
%T A211062 28127,36170,45624,57041,70350,85892,103748,124748,148174,175083,
%U A211062 205683,239800,277908,321224,368301,421093,479209,543193,613374
%N A211062 Number of 2 X 2 matrices having all terms in {1,...,n} and determinant >= 2n.
%C A211062 For a guide to related sequences, see A210000.
%H A211062 Chai Wah Wu, <a href="/A211062/b211062.txt">Table of n, a(n) for n = 1..1000</a>
%t A211062 a = 1; b = n; z1 = 40;
%t A211062 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A211062 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A211062 d[n_, m_] := d[n, m] = Sum[c[n, k], {k, 0, m}]
%t A211062 Table[d[n, n^2] - d[n, 2 n - 1], {n, 1, z1}]
%Y A211062 Cf. A210000.
%K A211062 nonn
%O A211062 1,3
%A A211062 _Clark Kimberling_, Mar 31 2012