cp's OEIS Frontend

A209994 Number of 2 X 2 matrices with all elements in {1,2,...,n} and determinant in {-1,0,1}.

%I A209994 #6 Jan 12 2017 01:39:48
%S A209994 0,1,10,31,60,105,154,223,300,393,490,607,748,889,1034,1215,1404,1593,
%T A209994 1818,2031,2300,2569,2810,3071,3436,3753,4042,4399,4796,5129,5610,
%U A209994 5967,6412,6857,7242,7759,8380,8809,9242,9775,10460
%N A209994 Number of 2 X 2 matrices with all elements in {1,2,...,n} and determinant in {-1,0,1}.
%C A209994 See A210000 for a guide to related sequences.
%t A209994 a = 1; b = n; z1 = 40;
%t A209994 t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
%t A209994 c[n_, k_] := c[n, k] = Count[t[n], k]
%t A209994 c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, -m, m}]
%t A209994 Table[c1[n, 1], {n, 0, z1}]    (* A209994 *)
%Y A209994 Cf. A210000.
%K A209994 nonn
%O A209994 0,3
%A A209994 _Clark Kimberling_, Mar 18 2012