Olympiad Maths Prep

Track / Stage 8 / 5 of 180 #1705 of 2000

Problem 1705

IMO Shortlist mid-range; USAMO P2/P5
Number theory Difficulty 8.0 Find the answer china_team_selection_test

For positive integer k>1k>1, let f(k)f(k) be the number of ways of factoring kk into product of positive integers greater than 11 (The order of factors are not countered, for example f(12)=4f(12)=4, as 1212 can be factored in these 44 ways: 12,26,34,22312,2\cdot 6,3\cdot 4, 2\cdot 2\cdot 3.
Prove: If nn is a positive integer greater than 11, pp is a prime factor of nn, then f(n)npf(n)\leq \frac{n}{p}

Official solution

For a positive integer k>1 k > 1 , let f(k) f(k) represent the number of ways to factor k k into a product of positive integers greater than 1. For example, f(12)=4 f(12) = 4 because 12 can be factored in these 4 ways: 12 12 , 26 2 \cdot 6 , 34 3 \cdot 4 , and 223 2 \cdot 2 \cdot 3 .

We aim to prove that if n n is a positive integer greater than 1 and p p is a prime factor of n n , then f(n)np f(n) \leq \frac{n}{p} .

We proceed by using strong induction. The base case is clear.

Let p p be the largest prime divisor of n n . We need to show that f(n)np f(n) \leq \frac{n}{p} .

Consider n=xj n = \prod x_j where each xj>1 x_j > 1 . Suppose one of the factors xi=pd1 x_i = p \cdot d_1 . Then d1 d_1 must divide np \frac{n}{p} , implying that:
f(n)d1npf(npd1). f(n) \leq \sum_{d_1 \mid \frac{n}{p}} f\left(\frac{n}{p d_1}\right).

By the inductive hypothesis, for any k<n k < n , we have f(k)kQ(k)ϕ(k) f(k) \leq \frac{k}{Q(k)} \leq \phi(k) , where Q(k) Q(k) is the largest prime factor of k k and ϕ(k) \phi(k) is the Euler's totient function.

Thus, we can write:
f(n)d1npf(npd1)d1npϕ(npd1)=np. f(n) \leq \sum_{d_1 \mid \frac{n}{p}} f\left(\frac{n}{p d_1}\right) \leq \sum_{d_1 \mid \frac{n}{p}} \phi\left(\frac{n}{p d_1}\right) = \frac{n}{p}.

Therefore, we have shown that f(n)np f(n) \leq \frac{n}{p} as required.

The answer is: \boxed{\frac{n}{p}}.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.