Let be a natural number $ n\ge 2 $ and an expression of $ n $ variables
$$ E\left( x_1,x_2,...,x_n\right) =x_1^2+x_2^2+\cdots +x_n^2-x_1x_2-x_2x_3-\cdots -x_{n-1}x_n -x_nx_1. $$
Determine $ \sup_{x_1,...,x_n\in [0,1]} E\left( x_1,x_2,...,x_n\right)  $ and the specific values at which this supremum is attained.
1. We start with the given expression:
   \[
   E(x_1, x_2, \ldots, x_n) = x_1^2 + x_2^2 + \cdots + x_n^2 - x_1 x_2 - x_2 x_3 - \cdots - x_{n-1} x_n - x_n x_1
   \]

2. We can rewrite \(E\) using the identity for the sum of squares:
   \[
   E = \frac{1}{2} \sum_{i=1}^n (x_i - x_{i+1})^2
   \]
   where \(x_{n+1} = x_1\) to handle the cyclic nature of the indices.

3. To find the supremum of \(E\) over \(x_i \in [0, 1]\), we need to maximize the expression \(\sum_{i=1}^n (x_i - x_{i+1})^2\).

4. Consider the case when \(n\) is even. Let \(n = 2k\). We can choose \(x_i\) such that \(x_i = 1\) for odd \(i\) and \(x_i = 0\) for even \(i\). This gives:
   \[
   E(1, 0, 1, 0, \ldots, 1, 0) = \frac{1}{2} \sum_{i=1}^n (x_i - x_{i+1})^2 = \frac{1}{2} \sum_{i=1}^n 1 = \frac{n}{2}
   \]
   This is achieved by any cyclic permutation of \((1, 0, 1, 0, \ldots, 1, 0)\).

5. Now consider the case when \(n\) is odd. Let \(n = 2k + 1\). We can choose \(x_i\) such that \(x_i = 1\) for odd \(i\) and \(x_i = 0\) for even \(i\). This gives:
   \[
   E(1, 0, 1, 0, \ldots, 1) = \frac{1}{2} \sum_{i=1}^n (x_i - x_{i+1})^2 = \frac{1}{2} \sum_{i=1}^{n-1} 1 + \frac{1}{2} (1 - 0)^2 = \frac{n-1}{2}
   \]
   This is achieved by any cyclic permutation of \((1, 0, 1, 0, \ldots, 1, 0, 0)\) or \((1, 0, 1, 0, \ldots, 1, 0, 1)\).

6. Therefore, the supremum of \(E\) is \(\frac{n}{2}\) for even \(n\) and \(\frac{n-1}{2}\) for odd \(n\).

The final answer is \(\boxed{\frac{n}{2} \text{ for even } n \text{ and } \frac{n-1}{2} \text{ for odd } n}\)