a(n) = Product_{1<=k<=n/2, gcd(k,n)=1} k.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =1a(6) =6a(7) =3a(8) =8a(9) =3a(10) =120a(11) =5a(12) =720a(13) =15a(14) =56a(15) =105a(16) =40320a(17) =35a(18) =362880a(19) =189a(20) =3200a(21) =945a(22) =39916800a(23) =385a(24) =9580032a(25) =10395a(26) =3203200a(27) =19305a(28) =87178291200a(29) =1001