%I #7 Nov 29 2016 20:57:29
%S 1,0,5,26,83,203,456,853,1497,2477,3860,5690,8305,11470,15684,20947,
%T 27328,35057,44569,55569,68849
%N Number of 2 X 2 matrices with all elements in {0,1,...,n} and determinant >= 2n.
%C See A210000 for a guide to related sequences.
%H Chai Wah Wu, <a href="/A210367/b210367.txt">Table of n, a(n) for n = 0..1000</a>
%e a(2)=5 counts these matrices:
%e 2 0...2 1...2 0...2 0...2 2
%e 0 2...0 2...1 2...2 2...0 2
%t a = 0; b = n; z1 = 20;
%t t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t c[n_, k_] := c[n, k] = Count[t[n], k]
%t c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
%t Table[c1[n, n^2] - c1[n, 2 n - 1], {n, 0, z1}]
%Y Cf. A210000.
%K nonn
%O 0,3
%A _Clark Kimberling_, Mar 20 2012