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Number theory Difficulty 6.8 National olympiad Find the answer

Let m>1m>1 be an integer. Find the smallest positive integer nn, such that for any integers a1,a2,,an;b1,b2,,bna_1,a_2,\ldots ,a_n; b_1,b_2,\ldots ,b_n there exists integers x1,x2,,xnx_1,x_2,\ldots ,x_n satisfying the following two conditions:

i) There exists i{1,2,,n}i\in \{1,2,\ldots ,n\} such that xix_i and mm are coprime

ii) i=1naixii=1nbixi0(modm)\sum^n_{i=1} a_ix_i \equiv \sum^n_{i=1} b_ix_i \equiv 0 \pmod m

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let m>1 m > 1 be an integer. We are tasked with finding the smallest positive integer n n such that for any integers a1,a2,,an a_1, a_2, \ldots, a_n and b1,b2,,bn b_1, b_2, \ldots, b_n , there exist integers x1,x2,,xn x_1, x_2, \ldots, x_n satisfying the following two conditions:

1. There exists i{1,2,,n} i \in \{1, 2, \ldots, n\} such that xi x_i and m m are coprime.
2. i=1naixii=1nbixi0(modm)\sum_{i=1}^n a_i x_i \equiv \sum_{i=1}^n b_i x_i \equiv 0 \pmod{m}.

To solve this, we need to determine the structure of m m . Let m=p1k1p2k2ptkt m = p_1^{k_1} p_2^{k_2} \cdots p_t^{k_t} , where p1,p2,,pt p_1, p_2, \ldots, p_t are distinct prime factors of m m and ω(m)=t \omega(m) = t is the number of distinct primes dividing m m .

We will show that the smallest n n satisfying the conditions is 2ω(m)+1 2\omega(m) + 1 .

### Construction for 2ω(m) 2\omega(m) :
Consider n=2ω(m) n = 2\omega(m) . Write m=p1k1p2k2ptkt m = p_1^{k_1} p_2^{k_2} \cdots p_t^{k_t} and let ω(m)=t \omega(m) = t . For each s=1,,t s = 1, \ldots, t , let ps p_s divide all of the ai a_i and bi b_i 's except for a2s1 a_{2s-1} and b2s b_{2s} . Consequently, x2s1 x_{2s-1} and x2s x_{2s} must both be divisible by ps p_s , so none of the xi x_i 's are coprime to m m . Thus, n=2ω(m) n = 2\omega(m) is not sufficient.

### Proof for 2ω(m)+1 2\omega(m) + 1 :
To prove that n=2ω(m)+1 n = 2\omega(m) + 1 is sufficient, we use the following claim:

Claim: For a prime p p and a positive integer k k , n n is p p -friendly if and only if n n is pk p^k -friendly.

Proof: (Sketch)
1. Base Case: For k=1 k = 1 , the claim is trivially true.
2. Inductive Step: Assume the claim holds for k1 k-1 . We need to show it holds for k k . We consider four cases based on the linear independence and zero properties of the vectors c c and d d modulo p p . By carefully constructing xi x_i and using the properties of linear combinations and modular arithmetic, we show that the conditions hold for pk p^k .

By the Chinese Remainder Theorem (CRT), we can reduce the problem to considering each prime power pk p^k dividing m m . Since the matrix formed by ai a_i and bi b_i has at most 2t 2t entries that do not work, there exists at least one xi x_i that is coprime to m m when n=2ω(m)+1 n = 2\omega(m) + 1 .

Therefore, the smallest positive integer n n satisfying the conditions is:
n=2ω(m)+1. n = 2\omega(m) + 1.

The answer is: 2ω(m)+1\boxed{2\omega(m) + 1}.

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