%I #17 Feb 24 2019 11:06:56
%S 2,5,5,9,7,13,9,16,12,19,13,24,15,25,19,29,19,33,21,36,25,37,25,44,28,
%T 43,31,48,31,53,33,54,37,55,39,63,39,61,43,68,43,71,45,72,51,73,49,82,
%U 52,81,55,84,55,89,59,92,61,91,61,102,63,97,69,103,69,107,69,108,73
%N Number of Abelian subgroups of the dihedral group with 2n elements.
%C The rank of the fundamental group with genus one of the D_2n; cobordism category in dimension 1+1, with D_2n the dihedral group of order 2n. - _C. Segovia_, Dec 05 2012
%H C. Segovia, <a href="http://arxiv.org/abs/1211.2144">The classifying space of the 1+1 dimensional G-cobordism category</a>, arXiv:1211.2144 [math.AT], 2012-2013.
%F a(n) = n + tau(n) if n is odd, (3/2)*n + tau(n) if n is even, where tau(n) = the number of divisors of n (A000005).
%t a[n_] := If[OddQ[n], n, 3n/2] + DivisorSigma[0, n];
%t Array[a, 69] (* _Jean-François Alcover_, Feb 24 2019 *)
%Y Cf. A062249, A007503.
%K nonn
%O 1,1
%A Ahmed Fares (ahmedfares(AT)my-deja.com), Jul 02 2001
%E Formula and more terms from _Vladeta Jovovic_, Jul 05 2001