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Algebra Difficulty 7.6 National olympiad, round 2 Find the answer

Determine which integers n>1n > 1 have the property that there exists an infinite sequence a1,a2,a3,a_1, a_2, a_3, \ldots of nonzero integers such that the equality ak+2a2k++nank=0a_k+2a_{2k}+\ldots+na_{nk}=0holds for every positive integer kk.

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

Solution

Consider the problem to determine which integers n>1 n > 1 have the property that there exists an infinite sequence a1,a2,a3, a_1, a_2, a_3, \ldots of nonzero integers satisfying the equality:

ak+2a2k++nank=0 a_k + 2a_{2k} + \ldots + na_{nk} = 0

for every positive integer k k .

### Step-by-Step Solution:

1. Express the Condition: For every positive integer k k , the condition given can be expressed as:

j=1n(jajk)=0 \sum_{j=1}^{n}(j \cdot a_{jk}) = 0

2. Simplify the Problem: Let us analyze a few specific cases of n n to understand the behavior:

- **Case n=2 n = 2 :**

For n=2 n = 2 , consider the condition:

ak+2a2k=0 a_k + 2a_{2k} = 0

This implies:

ak=2a2k a_k = -2a_{2k}

If we attempt to assign values for ak a_k and a2k a_{2k} , we find ak a_k must be in a strict ratio with a2k a_{2k} . For consistency across different k k , this creates a problematic sequence unless some terms are zero, conflicting with the nonzero integer requirement.

- **Generalize for n>2 n > 2 :**

For n3 n \geq 3 , we have:

ak+2a2k++nank=0 a_k + 2a_{2k} + \ldots + na_{nk} = 0

Here, the additional terms 3a3k,,nank 3a_{3k}, \ldots, na_{nk} provide more freedom in choosing ajk a_{jk} . It becomes possible to balance the equation by selecting integers ajk a_{jk} such that the weighted sum equals zero, allowing an infinite sequence of nonzero solutions.

3. Conclude the Argument: From examining specific cases, especially n=2 n = 2 , adding more terms allows more flexibility in balancing the sum, unlike n=2 n = 2 , which forces a consistent but nonzero-infeasible solution.

Thus, the integers n>1 n > 1 that satisfy the conditions of the problem are n>2 n > 2 .

### Final Answer:

n>2 \boxed{n > 2}

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