OFFSET
1,1
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.
EXAMPLE
The sequence of terms together with their prime indices of prime indices begins:
4: {{},{}}
6: {{},{1}}
8: {{},{},{}}
10: {{},{2}}
12: {{},{},{1}}
14: {{},{1,1}}
16: {{},{},{},{}}
18: {{},{1},{1}}
20: {{},{},{2}}
21: {{1},{1,1}}
22: {{},{3}}
24: {{},{},{},{1}}
26: {{},{1,2}}
28: {{},{},{1,1}}
30: {{},{1},{2}}
32: {{},{},{},{},{}}
34: {{},{4}}
35: {{2},{1,1}}
36: {{},{},{1},{1}}
38: {{},{1,1,1}}
MATHEMATICA
Select[Range[100], !PrimeQ[#]&&!And@@PrimeQ/@PrimePi/@First/@If[#==1, {}, FactorInteger[#]]&]
CROSSREFS
The restriction to odd terms is A330949.
Nonprime numbers n such that A330944(n) > 0.
Taking odds instead of nonprimes gives A330946.
The number of prime prime indices is given by A257994.
Primes of prime index are A006450.
Primes of nonprime index are A007821.
Products of primes of nonprime index are A320628.
The set S of numbers whose prime indices do not all belong to S is A324694.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 13 2020
STATUS
approved