a(n) = Product_{k=1..n, gcd(k,n)=1} (1+k).

Terms

a(0) =2a(1) =2a(2) =6a(3) =8a(4) =120a(5) =12a(6) =5040a(7) =384a(8) =12960a(9) =640a(10) =39916800a(11) =1152a(12) =6227020800a(13) =80640a(14) =5443200a(15) =10321920a(17) =290304a(19) =38707200a(20) =384758035200a(21) =6812467200a(23) =139345920