Multiply successively by 1 (once), 2 (twice), 3 (thrice), etc.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =12a(5) =36a(6) =108a(7) =432a(8) =1728a(9) =6912a(10) =27648a(11) =138240a(12) =691200a(13) =3456000a(14) =17280000a(15) =86400000a(16) =518400000a(17) =3110400000a(18) =18662400000a(19) =111974400000a(20) =671846400000