Number of index n subgroups of modular group PSL_2(Z).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =5a(5) =22a(6) =42a(7) =40a(8) =120a(9) =265a(10) =286a(11) =764a(12) =1729a(13) =2198a(14) =5168a(15) =12144a(16) =17034a(17) =37702a(18) =88958a(19) =136584a(20) =288270a(21) =682572a(22) =1118996a(23) =2306464a(24) =5428800a(25) =9409517a(26) =19103988a(27) =44701696a(28) =80904113a(29) =163344502