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.
EXAMPLE
The sequence of terms together with their prime indices begins:
1: {} 22: {1,5} 44: {1,1,5}
2: {1} 23: {9} 46: {1,9}
3: {2} 24: {1,1,1,2} 47: {15}
4: {1,1} 25: {3,3} 48: {1,1,1,1,2}
5: {3} 26: {1,6} 49: {4,4}
6: {1,2} 27: {2,2,2} 50: {1,3,3}
7: {4} 28: {1,1,4} 52: {1,1,6}
8: {1,1,1} 29: {10} 53: {16}
9: {2,2} 31: {11} 54: {1,2,2,2}
10: {1,3} 32: {1,1,1,1,1} 56: {1,1,1,4}
11: {5} 34: {1,7} 57: {2,8}
12: {1,1,2} 35: {3,4} 58: {1,10}
13: {6} 36: {1,1,2,2} 59: {17}
14: {1,4} 37: {12} 61: {18}
16: {1,1,1,1} 38: {1,8} 62: {1,11}
17: {7} 39: {2,6} 63: {2,2,4}
18: {1,2,2} 40: {1,1,1,3} 64: {1,1,1,1,1,1}
19: {8} 41: {13} 65: {3,6}
20: {1,1,3} 42: {1,2,4} 67: {19}
21: {2,4} 43: {14} 68: {1,1,7}
MATHEMATICA
Select[Range[100], Count[PrimePi/@First/@FactorInteger[#], _?PrimeQ]<=1&]
CROSSREFS
These are numbers n such that A279952(n) <= 1.
Numbers whose prime indices are not all prime are A330945.
Numbers with at least one prime prime index are A331386.
The set S of numbers with at most one prime index in S are A331784.
The set S of numbers with at most one distinct prime index in S are A331912.
Numbers with at most one prime prime index are A331914.
Numbers with exactly one prime prime index are A331915.
Numbers with exactly one distinct prime prime index are A331916.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Feb 08 2020
STATUS
approved