a(n) = binomial(phi(n+1),phi(n)).

Terms

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