a(n) = Product_{k=1..n} phi(prime(k)-1).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =16a(6) =64a(7) =512a(8) =3072a(9) =30720a(10) =368640a(11) =2949120a(12) =35389440a(13) =566231040a(14) =6794772480a(15) =149484994560