Maths Olympiad Prep

Library / /21 of 30

Number theory Difficulty 8.5 Shortlist Prove it Germany

Problem:

For nine distinct positive integers d1,d2,,d9d_{1}, d_{2}, \ldots, d_{9} we consider the polynomial P(n)=(n+d1)(n+d2)(n+d9)P(n)=\left(n+d_{1}\right)\left(n+d_{2}\right) \cdot \ldots \cdot\left(n+d_{9}\right).
One is to show that there exists an integer NN with the following property:
For all integers nNn \geq N the number P(n)P(n) is divisible by a prime greater than 20.

Solution

Solution:

According to the statement of the problem, we may assume N>0N>0 and d1<d2<<d9d_{1}<d_{2}<\ldots<d_{9}. Then k=d9d1=max1q<r9(drdq)k=d_{9}-d_{1}=\max _{1 \leq q<r \leq 9}\left(d_{r}-d_{q}\right). Let zNz \in \mathbb{N} be chosen such that 2z>k2^{z}>k.

Claim: With N=2z3z5z7z11z13z17z19zN=2^{z} \cdot 3^{z} \cdot 5^{z} \cdot 7^{z} \cdot 11^{z} \cdot 13^{z} \cdot 17^{z} \cdot 19^{z} a solution for NN has been found.

Proof: We assume that for nNn \geq N none of the factors of P(n)P(n) has a prime factor greater than 20, and consider first n+d1=2e1(1)3e2(1)19e8(1)n+d_{1}=2^{e_{1}^{(1)}} \cdot 3^{e_{2}^{(1)}} \cdot \ldots \cdot 19^{e_{8}^{(1)}}. Since nNn \geq N, there must then be at least one ej(1)e_{j}^{(1)} with ej(1)ze_{j}^{(1)} \geq z (1j81 \leq j \leq 8). Let the corresponding prime factor be pp. Thus n+d1n+d_{1} is divisible by pej(1)p^{e_{j}^{(1)}}. Since pej(1)2j(1)2z>k=max1q<r9(drdq)p^{e_{j}^{(1)}} \geq 2^{\ell_{j}^{(1)}} \geq 2^{z}>k=\max _{1 \leq q<r \leq 9}\left(d_{r}-d_{q}\right), none of the other numbers n+din+d_{i} (2i92 \leq i \leq 9) is divisible by pej(1)p^{e_{j}^{(1)}}, because otherwise did1d_{i}-d_{1} would be divisible by pjej(1)p_{j}^{e_{j}^{(1)}}. Thus ej(1)=max1i9(ej(i))e_{j}^{(1)}=\max _{1 \leq i \leq 9}\left(e_{j}^{(i)}\right). Now consider n+d2n+d_{2}: there, because d2>d1d_{2}>d_{1}, we find at least one ej(2)e_{j}^{(2)} with ej(2)>ej(1)e_{j}^{(2)}>e_{j}^{(1)} as well as ej(2)ze_{j}^{(2)} \geq z. Hence n+d2n+d_{2} also has an ej(2)e_{j}^{(2)} that is maximal with respect to all factors. This holds correspondingly for the other factors n+din+d_{i}.

Since, however, there are only 8 prime factors less than 20 for the 9 factors of the polynomial, a contradiction arises here by the pigeonhole principle. Therefore a further prime factor must occur, by which P(n)P(n) is divisible.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement translated into English from de; metadata (topic, difficulty) added by this project.