a(1)=1; for n > 1, a(n) is the smallest number that has n divisors and is coprime to a(n-1).

Terms

a(0) =1a(1) =2a(2) =9a(3) =8a(4) =81a(5) =20a(6) =729a(7) =40a(8) =441a(9) =80a(10) =59049a(11) =140a(12) =531441a(13) =320a(14) =3969a(15) =440a(16) =43046721a(17) =700a(18) =387420489a(19) =560a(20) =88209a(21) =5120a(22) =31381059609a(23) =1120a(24) =1185921a(25) =20480a(26) =53361a(27) =4160a(29) =2800a(31) =3080