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, with product A003963(n).
Also Heinz numbers of integer partitions whose product of parts is even (counted by A047967), where the Heinz number of an integer partition (y_1,...,y_k) is prime(y_1) * ... * prime(y_k).
EXAMPLE
The sequence of terms together with their prime indices begins:
3: {2}
6: {1,2}
7: {4}
9: {2,2}
12: {1,1,2}
13: {6}
14: {1,4}
15: {2,3}
18: {1,2,2}
19: {8}
21: {2,4}
24: {1,1,1,2}
26: {1,6}
27: {2,2,2}
28: {1,1,4}
29: {10}
30: {1,2,3}
33: {2,5}
35: {3,4}
36: {1,1,2,2}
MATHEMATICA
Select[Range[100], EvenQ[Times@@PrimePi/@If[#==1, {}, FactorInteger[#]][[All, 1]]]&]
PROG
(PARI) isok(n) = my(f=factor(n)[, 1]); !(prod(k=1, #f, primepi(f[k])) % 2); \\ Michel Marcus, Mar 22 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Mar 21 2019
STATUS
approved