Number of conjugacy class of index n subgroups in PSL_2 (ZZ).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =2a(5) =1a(6) =8a(7) =6a(8) =7a(9) =14a(10) =27a(11) =26a(12) =80a(13) =133a(14) =170a(15) =348a(16) =765a(17) =1002a(18) =2176a(19) =4682a(20) =6931a(21) =13740a(22) =31085a(23) =48652a(24) =96682a(25) =217152a(26) =362779a(27) =707590a(28) =1597130a(29) =2789797