OFFSET
1,15
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. Numbers whose prime indices have a common divisor > 1 are listed in A318978.
Note that A318978 includes also all odd primes and their powers, thus the only numbers for which a maximum such divisor is 1 are the powers of 2. Therefore A000079 gives the indices of zeros in this sequence. - Antti Karttunen, Dec 06 2021
LINKS
EXAMPLE
We have 5115 -> 165 -> 15 -> 3 -> 1, so a(5115) = 4.
MATHEMATICA
Table[Length[FixedPointList[#/Max[Select[Divisors[#], GCD@@PrimePi/@First/@FactorInteger[#]!=1&]]&, n]]-2, {n, 100}]
PROG
(PARI)
A327405(n) = (n / vecmax(select(d -> (1==d)||(gcd(apply(primepi, factor(d)[, 1]~))>1), divisors(n))));
A327406(n) = { my(u = A327405(n), k=0); while(u!=n, k++; n = u; u = A327405(n)); (k); }; \\ Antti Karttunen, Dec 06 2021
CROSSREFS
First appearance of n is A080696(n).
See link for additional cross-references.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Sep 21 2019
EXTENSIONS
Data section extended up to 105 terms by Antti Karttunen, Dec 06 2021
STATUS
approved