A356224 - OEIS

A356224

Number of divisors of n whose prime indices cover an initial interval of positive integers.

1, 2, 1, 3, 1, 3, 1, 4, 1, 2, 1, 5, 1, 2, 1, 5, 1, 4, 1, 3, 1, 2, 1, 7, 1, 2, 1, 3, 1, 4, 1, 6, 1, 2, 1, 7, 1, 2, 1, 4, 1, 3, 1, 3, 1, 2, 1, 9, 1, 2, 1, 3, 1, 5, 1, 4, 1, 2, 1, 7, 1, 2, 1, 7, 1, 3, 1, 3, 1, 2, 1, 10, 1, 2, 1, 3, 1, 3, 1, 5, 1, 2, 1, 5, 1, 2, 1

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

LINKS

Table of n, a(n) for n=1..87.

EXAMPLE

The a(n) gapless divisors of n = 1..24:

1 2 1 4 1 6 1 8 1 2 1 12 1 2 1 16 1 18 1 4 1 2 1 24

1 2 2 4 1 6 1 8 6 2 1 12

1 1 2 4 4 2 1 8

1 2 2 1 6

1 1 4

For example, the divisors of 12 are {1,2,3,4,6,12}, of which {1,2,4,6,12} belong to A055932, so a(12) = 5.

MATHEMATICA

primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];

normQ[m_]:=m=={}||Union[m]==Range[Max[m]];

Table[Length[Select[Divisors[n], normQ[primeMS[#]]&]], {n, 100}]

CROSSREFS

These divisors belong to A055932, a subset of A073491 (complement A073492).

The complement is A356225.

A001223 lists the prime gaps.

A328338 has third-largest divisor prime.

A356226 gives the lengths of maximal gapless intervals of prime indices.

Cf. A000005, A001222, A028334, A029709, A055874, A056239, A070824, A112798, A119313, A137921, A287170, A289509, A356223.

Sequence in context: A135732 A342241 A322584 * A364767 A326154 A306248

Adjacent sequences: A356221 A356222 A356223 * A356225 A356226 A356227

KEYWORD

nonn

AUTHOR

Gus Wiseman, Aug 04 2022

STATUS

approved