OFFSET
1,6
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.
Is the image missing only 2 and 4?
EXAMPLE
The prime indices of 15 are {2,3}, with sums of pairs:
2+2 = 4
2+3 = 5
3+3 = 6
so a(15) = 3.
The prime indices of 180 are {1,1,2,2,3}, with sums of pairs:
1+1 = 2
1+2 = 3
1+3 = 4
2+2 = 4
2+3 = 5
3+3 = 6
so a(180) = 5.
MATHEMATICA
prix[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Table[Length[Union[Total/@Tuples[prix[n], 2]]], {n, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Nov 06 2023
STATUS
approved