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%I #8 Jan 01 2020 16:57:42
%S 1,1,1,2,1,4,1,4,4,4,2,10,1,5,7,12,1,10,4,10,12,4,6,24,4,12,7,10,5,26,
%T 6,20,13,12,7,30,11,4,15,32,6,26,6,28,23,12,7,52,12,10,28,10,6,40,14,
%U 44,17,24,12,56,19,12,20,36,27,44,11,28,16
%N Number of elements (a b, c d) in GL(2,Z) with det = -1, trace n and 0 <= a <= {b, c} <= d.
%H Sean A. Irvine, <a href="/A007104/b007104.txt">Table of n, a(n) for n = 1..250</a>
%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, D(i*(n-i)+1, i, n-i))} \\ _Andrew Howroyd_, Jan 01 2020
%Y Cf. A006546, A007203.
%K nonn
%O 1,4
%A John Lewis (frc.mass.edu)
%E More terms from _Sean A. Irvine_, Oct 12 2017
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