Binomial(n, phi(n)), where phi(n) is the Euler totient function.

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =5a(5) =15a(6) =7a(7) =70a(8) =84a(9) =210a(10) =11a(11) =495a(12) =13a(13) =3003a(14) =6435a(15) =12870a(16) =17a(17) =18564a(18) =19a(19) =125970a(20) =293930a(21) =646646a(22) =23a(23) =735471a(24) =53130a(25) =9657700a(26) =4686825a(27) =30421755a(28) =29a(29) =5852925