a(n) = exp(-Sum_{k=1..n} Sum_{d|k, d prime} moebius(d)*log(k/d)).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =2a(6) =12a(7) =12a(8) =48a(9) =144a(10) =1440a(11) =1440a(12) =34560a(13) =34560a(14) =483840a(15) =7257600a(16) =58060800a(17) =58060800a(18) =3135283200a(19) =3135283200a(20) =125411328000