A211059 - OEIS

%I #6 Jan 11 2017 01:31:43

%S 0,5,33,112,288,605,1145,1968,3176,4861,7161,10152,14040,18917,24961,

%T 32352,41312,51949,64585,79320,96472,116277,139025,164840,194184,

%U 227261,264385,305840,352096,403245,459945,522312,590840,665917

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

%C For a guide to related sequences, see A210000.

%t a = 1; b = n; z1 = 35;

%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] - c[n, 0], {n, 1, z1}]   (* A211059 *)

%t 2*%     (* A211056 *)

%Y Cf. A210000, A211056.

%K nonn

%O 1,2

%A _Clark Kimberling_, Mar 31 2012