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%I #13 Jan 31 2019 03:26:30
%S 1,2,6,28,192,1952,28800,618496,19132416,853508096,54584672256,
%T 5007197863936,656391329021952,122997864673574912,
%U 32863344789678981120,12522141669213840867328,6792468641646224590503936,5245768641385222636392415232
%N Number of elements of order <= 2 in group of n X n upper triangular matrices over GF(2).
%H I. M. Isaacs and Dikran B. Karagueuzian, <a href="https://doi.org/10.1090/S0025-5718-05-01705-9">Involutions and characters of upper triangular matrix groups</a>, Math. Comp., 74 (2005), 2027-2033.
%F a(n) = Sum_{r=0..floor(n/2)} f(n, r) where f(n, 0) = 1, f(1, r) = 0, f(n, r) = 2^r * f(n - 1, r) + (2^(n-r) - 2^(r-1)) * f(n - 1, r - 1). - _Sean A. Irvine_, Apr 15 2018
%K nonn
%O 1,2
%A Dikran Karagueuzian (dikran(AT)math.wisc.edu)
%E More terms from _Sean A. Irvine_, Apr 15 2018
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