OFFSET
1,2
COMMENTS
Let rad(n) be the radical of n, which equals the product of all prime factors of n (A007947). Let f(n) = Sum_{d | n} 1/rad(n). The sequence a(n) lists the denominators of the fractions f(n) in lowest terms.
f is a multiplicative function whose value on a prime power is f(p^k) = 1 + k/p. Hence f is a weighted divisor-counting function that weights divisors d higher when they have few and small prime divisors themselves.
EXAMPLE
The divisors of 12 are 1,2,3,4,6,12, so f(12) = 1 + (1/2) + (1/3) + (1/2) + (1/6) + (1/6) = 8/3, and a(12) = 3. Alternately, since f is multiplicative, f(12) = f(4)*f(3) = (1+2/2)*(1+1/3) = 8/3. The denominator of f(12) is 3 hence a(12) = 3.
MATHEMATICA
Array[Denominator@ DivisorSum[#, 1/Apply[Times, FactorInteger[#][[All, 1]] ] &] &, 80] (* Michael De Vlieger, Jan 19 2019 *)
PROG
(PARI) a(n) = denominator(sumdiv(n, d, 1/factorback(factor(d)[, 1]))) \\ David A. Corneth, Jan 01 2019
CROSSREFS
KEYWORD
frac,nonn
AUTHOR
David S. Metzler, Dec 31 2018
STATUS
approved