a(n) = d(d(d(n))), the 3rd iterate of the number-of-divisors function with an initial value of n.

Terms

a(0) =1a(1) =2a(2) =2a(3) =2a(4) =2a(5) =2a(6) =2a(7) =2a(8) =2a(9) =2a(10) =2a(11) =3a(12) =2a(13) =2a(14) =2a(15) =2a(16) =2a(17) =3a(18) =2a(19) =3a(20) =2a(21) =2a(22) =2a(23) =3a(24) =2a(25) =2a(26) =2a(27) =3a(28) =2a(29) =3