Number of subgroups of dihedral group: sigma(n) + d(n).

Terms

a(0) =2a(1) =5a(2) =6a(3) =10a(4) =8a(5) =16a(6) =10a(7) =19a(8) =16a(9) =22a(10) =14a(11) =34a(12) =16a(13) =28a(14) =28a(15) =36a(16) =20a(17) =45a(18) =22a(19) =48a(20) =36a(21) =40a(22) =26a(23) =68a(24) =34a(25) =46a(26) =44a(27) =62a(28) =32a(29) =80