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%I #12 May 05 2020 17:10:58
%S 0,20,52,116,180,308,372,564,692,884,1012,1332,1460,1844,2036,2292,
%T 2548,3060,3252,3828,4084,4468,4788,5492,5748,6388,6772,7348,7732,
%U 8628,8884,9844,10356,10996,11508,12276,12660,13812,14388,15156
%N Number of 2 X 2 matrices having all elements in {-n,...,n} and determinant 1.
%C See A210000 for a guide to related sequences.
%H Andrew Howroyd, <a href="/A209982/b209982.txt">Table of n, a(n) for n = 0..1000</a>
%F From _Andrew Howroyd_, May 05 2020: (Start)
%F a(n) = 8*A196227(n) + 4*(4*n + 1) = 8*A171503(n) - 4 for n > 0.
%F a(n) = -12 + 32*Sum_{k=1..n} phi(k) for n > 0. (End)
%t (See the Mathematica section at A209981.)
%o (PARI) a(n)=if(n<1, 0, 32*sum(k=1, n, eulerphi(k)) - 12) \\ _Andrew Howroyd_, May 05 2020
%Y Cf. A171503, A196227, A210000.
%K nonn
%O 0,2
%A _Clark Kimberling_, Mar 17 2012
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