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A197168 Number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is 2.

%I A197168 #14 Nov 29 2016 03:00:32
%S A197168 0,0,2,6,18,30,50,70,98,118,162,198,226,270,338,366,426,486,554,622,
%T A197168 682,726,842,926,986,1062,1202,1270,1362,1470,1562,1678,1802,1878,
%U A197168 2066,2158,2250,2390,2602,2694,2818,2974,3114,3278,3434,3526,3786,3966,4090,4254,4490
%N A197168 Number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is 2.
%C A197168 It is also the number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is -2.
%H A197168 Chai Wah Wu, <a href="/A197168/b197168.txt">Table of n, a(n) for n = 0..10000</a>
%F A197168 a(n) = A209973(n) - 4*n - 2 for n >= 2. - _Chai Wah Wu_, Nov 28 2016
%t A197168 Table[cnt = 0; Do[If[a*d-b*c == 2, cnt++], {a, n}, {b, n}, {c, n}, {d, n}]; cnt, {n, 50}] (* _T. D. Noe_, Oct 11 2011 *)
%K A197168 nonn
%O A197168 0,3
%A A197168 _Aldo González Lorenzo_, Oct 10 2011
%E A197168 a(0) added by _Chai Wah Wu_, Nov 27 2016