a(n) = n-th n-almost prime.

Terms

a(0) =2a(1) =6a(2) =18a(3) =40a(4) =108a(5) =224a(6) =480a(7) =1296a(8) =2688a(9) =5632a(10) =11520a(11) =25600a(12) =53248a(13) =124416a(14) =258048a(15) =540672a(16) =1105920a(17) =2228224a(18) =4587520a(19) =9830400a(20) =19922944a(21) =40894464a(22) =95551488a(23) =192937984a(24) =396361728a(25) =822083584a(26) =1660944384a(27) =3397386240a(28) =6845104128