Genus of modular group Gamma_0(n). Or, genus of modular curve X_0(n).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =0a(7) =0a(8) =0a(9) =0a(10) =1a(11) =0a(12) =0a(13) =1a(14) =1a(15) =0a(16) =1a(17) =0a(18) =1a(19) =1a(20) =1a(21) =2a(22) =2a(23) =1a(24) =0a(25) =2a(26) =1a(27) =2a(28) =2a(29) =3