Determine the polynomial $P\in \mathbb{R}[x]$ for which there exists $n\in \mathbb{Z}_{>0}$ such that for all $x\in \mathbb{Q}$ we have: \[P\left(x+\frac1n\right)+P\left(x-\frac1n\right)=2P(x).\]

[i]Dumitru Bușneag[/i]
1. Let's start by considering the given functional equation for the polynomial \( P(x) \):
   \[
   P\left(x+\frac{1}{n}\right) + P\left(x-\frac{1}{n}\right) = 2P(x)
   \]
   for all \( x \in \mathbb{Q} \) and some \( n \in \mathbb{Z}_{>0} \).

2. Assume \( P(x) \) is a polynomial of degree \( d \). We can write:
   \[
   P(x) = a_d x^d + a_{d-1} x^{d-1} + \cdots + a_1 x + a_0
   \]

3. Substitute \( x + \frac{1}{n} \) and \( x - \frac{1}{n} \) into \( P(x) \):
   \[
   P\left(x + \frac{1}{n}\right) = a_d \left(x + \frac{1}{n}\right)^d + a_{d-1} \left(x + \frac{1}{n}\right)^{d-1} + \cdots + a_1 \left(x + \frac{1}{n}\right) + a_0
   \]
   \[
   P\left(x - \frac{1}{n}\right) = a_d \left(x - \frac{1}{n}\right)^d + a_{d-1} \left(x - \frac{1}{n}\right)^{d-1} + \cdots + a_1 \left(x - \frac{1}{n}\right) + a_0
   \]

4. Add these two expressions:
   \[
   P\left(x + \frac{1}{n}\right) + P\left(x - \frac{1}{n}\right) = a_d \left[\left(x + \frac{1}{n}\right)^d + \left(x - \frac{1}{n}\right)^d\right] + a_{d-1} \left[\left(x + \frac{1}{n}\right)^{d-1} + \left(x - \frac{1}{n}\right)^{d-1}\right] + \cdots + 2a_0
   \]

5. For the equation \( P\left(x + \frac{1}{n}\right) + P\left(x - \frac{1}{n}\right) = 2P(x) \) to hold, each term in the polynomial must satisfy the equation. This implies:
   \[
   a_d \left[\left(x + \frac{1}{n}\right)^d + \left(x - \frac{1}{n}\right)^d\right] = 2a_d x^d
   \]
   \[
   a_{d-1} \left[\left(x + \frac{1}{n}\right)^{d-1} + \left(x - \frac{1}{n}\right)^{d-1}\right] = 2a_{d-1} x^{d-1}
   \]
   \[
   \vdots
   \]
   \[
   2a_0 = 2a_0
   \]

6. Consider the highest degree term \( a_d x^d \). The equation:
   \[
   \left(x + \frac{1}{n}\right)^d + \left(x - \frac{1}{n}\right)^d = 2x^d
   \]
   must hold for all \( x \in \mathbb{Q} \). This is only possible if \( d \leq 1 \), because for \( d \geq 2 \), the left-hand side will not simplify to \( 2x^d \).

7. Therefore, the polynomial \( P(x) \) must be of degree at most 1. Let:
   \[
   P(x) = ax + b
   \]

8. Verify that \( P(x) = ax + b \) satisfies the given functional equation:
   \[
   P\left(x + \frac{1}{n}\right) = a\left(x + \frac{1}{n}\right) + b = ax + \frac{a}{n} + b
   \]
   \[
   P\left(x - \frac{1}{n}\right) = a\left(x - \frac{1}{n}\right) + b = ax - \frac{a}{n} + b
   \]
   \[
   P\left(x + \frac{1}{n}\right) + P\left(x - \frac{1}{n}\right) = \left(ax + \frac{a}{n} + b\right) + \left(ax - \frac{a}{n} + b\right) = 2ax + 2b = 2P(x)
   \]

9. Hence, the polynomial \( P(x) = ax + b \) satisfies the given condition.

\(\blacksquare\)

The final answer is \( \boxed{ P(x) = ax + b } \)