%I #14 Aug 09 2023 12:55:08
%S 1,6,55,234,466,758,1704,24587,6656,6718,6428,114210,17158,26180,
%T 135869,701426,33277,126920,52320,1341656,469872,87924,48976,50257306,
%U 140918,291060,348528,3952920,175438,3685062,285952,12024029,1657948,567994,4624192,15262640,539046,838096
%N Number of subgroups of the group GL(2,Z_n) of invertible 2 X 2 matrices mod n (sequence A000252).
%o (GAP) c := CyclicGroup(n); aut := AutomorphismGroup(DirectProduct(c,c)); lat := LatticeSubgroups(aut);
%o (Magma) [&+[s`length : s in Subgroups(GL(2,ResidueClassRing(n)))] : n in [2..10]]; // _Robin Visser_, Aug 09 2023
%Y Cf. A000252.
%K nonn
%O 1,2
%A Sharon Sela (sharonsela(AT)hotmail.com), Jan 04 2002
%E a(7)-a(11) from _Andrew Howroyd_, Jul 02 2018
%E More terms from _Robin Visser_, Aug 09 2023