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Number theory Difficulty 7.2 National olympiad, round 2 Prove it

Example 2 Proof: There exists an increasing sequence of positive integers {an}\left\{a_{n}\right\}, such that for any kNk \in \mathbf{N}^{*}, the sequence {k+an}\left\{k+a_{n}\right\} contains at most a finite number of prime numbers.

Solution

Let p1,p2,p_{1}, p_{2}, \cdots denote all prime numbers in ascending order.
Now, we construct the sequence {an}\left\{a_{n}\right\} that meets the requirements.
Let a1=2a_{1}=2, and suppose a1,a2,,ana_{1}, a_{2}, \cdots, a_{n} have been determined. Take an+1a_{n+1} as the smallest positive integer greater than ana_{n} that satisfies the following system of congruences:
{x0(modp1),x1(modp2),xn(modpn+1)\left\{\begin{array}{l} x \equiv 0\left(\bmod p_{1}\right), \\ x \equiv-1\left(\bmod p_{2}\right), \\ \cdots \\ x \equiv-n\left(\bmod p_{n+1}\right) \end{array}\right.

Note that the Chinese Remainder Theorem guarantees the existence of such an an+1a_{n+1}.
We claim that the sequence {an}\left\{a_{n}\right\} defined by the above recursion satisfies the problem's requirements. For any kNk \in \mathbf{N}^{*}, when nk+1n \geqslant k+1, we have k+an0(modpk+1)k+a_{n} \equiv 0\left(\bmod p_{k+1}\right). Combining this with the fact that {an}\left\{a_{n}\right\} is increasing, it follows that every term of {k+an}\left\{k+a_{n}\right\} from the (k+2)(k+2)-th term onward is a multiple of pk+1p_{k+1} and is greater than pk+1p_{k+1}. Therefore, {k+an}\left\{k+a_{n}\right\} has at most k+1k+1 prime terms.

The proposition is thus 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.