Calculate the determinant of the $ n\times n $ complex matrix $ \left(a_j^i\right)_{1\le j\le n}^{1\le i\le n} $ defined by
$$ a_j^i=\left\{\begin{matrix} 1+x^2,\quad i=j\\x,\quad |i-j|=1\\0,\quad |i-j|\ge 2\end{matrix}\right. , $$
where $ n $ is a natural number greater than $ 2. $
To calculate the determinant of the given \( n \times n \) complex matrix \( \left(a_j^i\right)_{1 \le j \le n}^{1 \le i \le n} \), we need to understand the structure of the matrix and use properties of determinants.

1. **Matrix Definition and Structure**:
   The matrix \( A_n \) is defined by the elements:
   \[
   a_j^i = \begin{cases} 
   1 + x^2 & \text{if } i = j \\
   x & \text{if } |i - j| = 1 \\
   0 & \text{if } |i - j| \ge 2 
   \end{cases}
   \]
   This means \( A_n \) is a tridiagonal matrix with \( 1 + x^2 \) on the main diagonal, \( x \) on the subdiagonal and superdiagonal, and zeros elsewhere.

2. **Determinant Calculation**:
   We will use the recurrence relation for the determinant of a tridiagonal matrix. Let \( D_n \) denote the determinant of \( A_n \).

3. **Base Cases**:
   For \( n = 1 \):
   \[
   D_1 = 1 + x^2
   \]
   For \( n = 2 \):
   \[
   D_2 = \begin{vmatrix}
   1 + x^2 & x \\
   x & 1 + x^2
   \end{vmatrix} = (1 + x^2)(1 + x^2) - x^2 = 1 + 2x^2 + x^4 - x^2 = 1 + x^2 + x^4
   \]

4. **Recurrence Relation**:
   For \( n \ge 3 \), we expand the determinant along the first row:
   \[
   D_n = (1 + x^2)D_{n-1} - x^2 D_{n-2}
   \]
   This recurrence relation can be derived by considering the cofactor expansion along the first row and noting the tridiagonal structure.

5. **Induction Hypothesis**:
   Assume \( D_k = \sum_{i=0}^k x^{2i} \) holds for \( k = n-1 \) and \( k = n-2 \).

6. **Inductive Step**:
   Using the recurrence relation:
   \[
   D_n = (1 + x^2)D_{n-1} - x^2 D_{n-2}
   \]
   Substitute the induction hypothesis:
   \[
   D_n = (1 + x^2) \sum_{i=0}^{n-1} x^{2i} - x^2 \sum_{i=0}^{n-2} x^{2i}
   \]
   Simplify the expression:
   \[
   D_n = \sum_{i=0}^{n-1} x^{2i} + x^2 \sum_{i=0}^{n-1} x^{2i} - x^2 \sum_{i=0}^{n-2} x^{2i}
   \]
   \[
   D_n = \sum_{i=0}^{n-1} x^{2i} + \sum_{i=1}^{n} x^{2i} - \sum_{i=1}^{n-1} x^{2i}
   \]
   \[
   D_n = 1 + \sum_{i=1}^{n-1} x^{2i} + x^{2n} = \sum_{i=0}^{n} x^{2i}
   \]
   Thus, the induction step holds, and by mathematical induction, \( D_n = \sum_{i=0}^n x^{2i} \) for all \( n \ge 1 \).

The final answer is \(\boxed{\sum_{k=0}^n x^{2k}}\).