OFFSET
1,5
COMMENTS
Zeros are ignored when computing GCD, and the empty set has GCD 0.
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
85 has prime indices {3,7}, so a(85) = GCD(2,6) = 2.
MATHEMATICA
Table[GCD@@(PrimePi/@First/@If[n==1, {}, FactorInteger[n]]-1), {n, 100}]
CROSSREFS
Positions of 0's are A000079.
Positions of 1's are A328168.
Positions of records (first appearances) are A006005.
The GCD of the prime indices of n is A289508(n).
The GCD of the prime indices of n, all plus 1, is A328169(n).
Looking at divisors instead of prime indices gives A258409.
Partitions whose parts minus 1 are relatively prime are A328170.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 08 2019
STATUS
approved