OFFSET
1,2
COMMENTS
A prime index of n is a number m such that prime(m) divides n.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
Sum_{n>=1} 1/a(n) = 1 + Sum_{p in A006450} 1/(p-1). - Amiram Eldar, Sep 19 2022
EXAMPLE
Entry A302242 describes a correspondence between positive integers and multiset multisystems. In this case it gives the following sequence of set multisystems.
001: {}
003: {{1}}
005: {{2}}
009: {{1},{1}}
011: {{3}}
017: {{4}}
025: {{2},{2}}
027: {{1},{1},{1}}
031: {{5}}
041: {{6}}
059: {{7}}
067: {{8}}
081: {{1},{1},{1},{1}}
083: {{9}}
109: {{10}}
121: {{3},{3}}
125: {{2},{2},{2}}
MATHEMATICA
Select[Range[400], MatchQ[FactorInteger[#], {{_?(#===1||PrimeQ[PrimePi[#]]&), _}}]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Apr 10 2018
STATUS
approved