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Number theory Difficulty 7.0 National olympiad Prove it

Example 1 Let mm be a given positive integer, and each term of the sequence {an}\left\{a_{n}\right\} is a positive integer, and for any nNn \in \mathbf{N}^{*}, we have 0<an+1anm0<a_{n+1}-a_{n} \leqslant m. Prove: There exist infinitely many pairs of positive integers (p,q)(p, q), such that p<qp<q, and apaqa_{p} \mid a_{q}.

Solution

Proof
First, we prove that there exists a pair (p,q)(p, q) that satisfies the condition.
Consider the following number table:
x0,1,x0,2,,x0,mx1,1,x1,2,,x1,mxm,1,xm,2,,xm,m\begin{array}{l} x_{0,1}, x_{0,2}, \cdots, x_{0, m} \\ x_{1,1}, x_{1,2}, \cdots, x_{1, m} \\ \cdots \\ x_{m, 1}, x_{m, 2}, \cdots, x_{m, m} \end{array}

where x0,1=a1,x0,jx0,j1+1,j=2,3,,mx_{0,1}=a_{1}, x_{0, j} \rightleftharpoons x_{0, j-1}+1, j=2,3, \cdots, m. And
xi,j=(k=1mxi1,k)+xi1,j,1i,jmx_{i, j}=\left(\prod_{k=1}^{m} x_{i-1, k}\right)+x_{i-1, j}, 1 \leqslant i, j \leqslant m

The numbers in the above table have the following properties: each row from left to right is exactly a sequence of mm consecutive positive integers; in any two numbers a,ba, b in each column, the smaller number (let it be aa) is a divisor of the larger number, i.e., aba \mid b.

Since 0<an+1anm0 < a_{n+1} - a_{n} \leqslant m, each row of the table contains at least two terms from {an}\{a_{n}\}, so the table contains at least 2(m+1)2(m+1) numbers {ann=1,2,}\in \{a_{n} \mid n=1,2, \cdots\}. Therefore, there must be a column in the table that contains two numbers simultaneously belonging to {ann=1,2,}\{a_{n} \mid n=1,2, \cdots\}, denoted as ap,aq(p<q)a_{p}, a_{q} (p<q), then apaqa_{p} \mid a_{q}. Thus, we have found a pair (p,q)(p, q) that satisfies the condition.

Now, set x0,1x_{0,1} to aq+1a_{q}+1, and construct a number table with the same properties. We can find another pair of numbers (p,q)\left(p^{\prime}, q^{\prime}\right) that satisfy the condition. By continuing this process, we can find infinitely many pairs (p,q)(p, q) such that p<qp<q and apaqa_{p} \mid a_{q}.

The proposition is proved.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.