Array T(n,k) = number of subgroups of index k in free group of rank n, read by antidiagonals.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =1a(6) =1a(7) =7a(8) =13a(9) =1a(10) =1a(11) =15a(12) =97a(13) =71a(14) =1a(15) =1a(16) =31a(17) =625a(18) =2143a(19) =461a(20) =1a(21) =1a(22) =63a(23) =3841a(24) =54335a(25) =68641a(26) =3447a(27) =1a(28) =1a(29) =127