a(n) is the A106490 index where n first occurs.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =36a(5) =144a(6) =720a(7) =3600a(8) =25200a(9) =176400a(10) =1587600a(11) =17463600a(12) =192099600a(13) =2497294800a(14) =32464832400a(15) =551902150800