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%I #5 Jan 01 2020 16:57:26
%S 1,1,2,4,3,7,4,8,6,12,5,14,7,13,13,20,5,20,10,24,14,22,12,28,18,30,14,
%T 22,17,48,12,28,19,43,19,42,25,32,23,60,18,44,28,36,33,38,24,80,18,60,
%U 39,46,18,58,36,72,29,68,28,76,37,64,34,64,61,72,25,56,34,108
%N Number of elements (a b / c d) in GL(2,Z) with |det| = 1, trace n and 0 <= a <= {b, c} <= d.
%H Andrew Howroyd, <a href="/A007203/b007203.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = A006546(n) + A007104(n). - _Andrew Howroyd_, Jan 01 2020
%o (PARI)
%o D(n,s,t)={if(n>=1, sumdiv(n, d, s<=min(d,n/d) && max(d,n/d)<=t), 0)}
%o a(n)={sum(i=0, n\2, my(ad=i*(n-i)); D(ad+1, i, n-i) + D(ad-1, i, n-i))} \\ _Andrew Howroyd_, Jan 01 2020
%Y Cf. A006546, A007104, A007295.
%K nonn
%O 1,3
%A John Lewis (frc.mass.edu)
%E Terms a(41) and beyond from _Andrew Howroyd_, Jan 01 2020
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