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GCD of the prime indices of n, all minus 1.
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%I #7 Oct 13 2019 11:19:31

%S 0,0,1,0,2,1,3,0,1,2,4,1,5,3,1,0,6,1,7,2,1,4,8,1,2,5,1,3,9,1,10,0,1,6,

%T 1,1,11,7,1,2,12,1,13,4,1,8,14,1,3,2,1,5,15,1,2,3,1,9,16,1,17,10,1,0,

%U 1,1,18,6,1,1,19,1,20,11,1,7,1,1,21,2,1,12

%N GCD of the prime indices of n, all minus 1.

%C Zeros are ignored when computing GCD, and the empty set has GCD 0.

%C 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.

%e 85 has prime indices {3,7}, so a(85) = GCD(2,6) = 2.

%t Table[GCD@@(PrimePi/@First/@If[n==1,{},FactorInteger[n]]-1),{n,100}]

%Y Positions of 0's are A000079.

%Y Positions of 1's are A328168.

%Y Positions of records (first appearances) are A006005.

%Y The GCD of the prime indices of n is A289508(n).

%Y The GCD of the prime indices of n, all plus 1, is A328169(n).

%Y Looking at divisors instead of prime indices gives A258409.

%Y Partitions whose parts minus 1 are relatively prime are A328170.

%Y Cf. A000837, A001222, A007359, A056239, A112798, A289509, A318978, A318981, A328163, A328164.

%K nonn

%O 1,5

%A _Gus Wiseman_, Oct 08 2019