A355030 a(n) is the number of possible values of the number of prime divisors (counted with multiplicity) of numbers with n divisors.

1, 1, 1, 2, 1, 2, 1, 3, 2, 2, 1, 4, 1, 2, 2, 5, 1, 4, 1, 4, 2, 2, 1, 7, 2, 2, 3, 4, 1, 5, 1, 7, 2, 2, 2, 8, 1, 2, 2, 6, 1, 5, 1, 4, 4, 2, 1, 11, 2, 4, 2, 4, 1, 7, 2, 7, 2, 2, 1, 11, 1, 2, 4, 11, 2, 5, 1, 4, 2, 5, 1, 14, 1, 2, 4, 4, 2, 5, 1, 11, 5, 2, 1, 11, 2

Offset

1,4

Comments

First differs from A305254 at n = 40, from A001055 and A252665 at n = 36, from A218320 at n = 32 and from A317791, A318559 and A326334 at n = 30.

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

a(n) <= A001055(n).

a(p) = 1 for p prime.

a(A355031(n)) = n.

Example

a(2) = 1 since numbers with 2 divisors are primes, i.e., numbers k with the single value Omega(k) = 1.
a(4) = 2 since numbers with 4 divisors are either of the following 2 forms: p1 * p2 with p1 and p2 being distinct primes, or of the form p^3 with p prime.
a(8) = 3 since numbers with 8 divisors are either of the following 3 forms: p1 * p2 * p3 with p1, p2 and p3 being distinct primes, p1 * p2^3, or p1^7.

Mathematica

Table[Length[Union[Total[#-1]& /@ f[n]]], {n, 1, 100}] (* using the function f by T. D. Noe at A162247 *)

Crossrefs

Row lengths of A355029.

Cf. A000005, A001222, A118914, A162247, A355027.

Cf. A001055, A218320, A305254, A317791, A318559, A326334.

Sequence in context: A318559 A326334 A218320 * A305254 A252665 A001055

Adjacent sequences: A355027 A355028 A355029 * A355031 A355032 A355033

Keyword

nonn

Author

Amiram Eldar, Jun 16 2022

Status

approved