Eigenvalues and inverse of a Toeplitz matrix

Question

Given the Toeplitz matrix

$$X = \begin{pmatrix} ~~~~\textbf{1} ~~~~\textbf{c} ~~~~\textbf{d} ~~~~0 ~~~~0 ~~~~0 ~~~~\textbf{d}~~~~ \\ ~~~~\textbf{c} ~~~~\textbf{1} ~~~~\textbf{c} ~~~~\textbf{d} ~~~~0 ~~~~0 ~~~~0~~~~ \\ ~~~~\textbf{d} ~~~~\textbf{c} ~~~~\textbf{1} ~~~~\textbf{c} ~~~~\textbf{d} ~~~~0 ~~~~0~~~~ \\ ~~~~0 ~~~~\textbf{d} ~~~~\textbf{c} ~~~~\textbf{1} ~~~~\textbf{c} ~~~~\textbf{d} ~~~~0~~~~ \\ ~~~~0 ~~~~0 ~~~~\textbf{d} ~~~~\textbf{c} ~~~~\textbf{1} ~~~~\textbf{c} ~~~~\textbf{d}~~~~ \\ ~~~~0 ~~~~0 ~~~~0 ~~~~\textbf{d} ~~~~\textbf{c} ~~~~\textbf{1} ~~~\textbf{c}~~~~ \\ ~~~~\textbf{d} ~~~~0 ~~~~0 ~~~~0 ~~~~\textbf{d} ~~~~\textbf{c} ~~~~\textbf{1}~~~~ \\ \end{pmatrix}$$

where $d$ and $c$ are different values between $-1$ and $0$. Furthermore,

$$2c + 2d = -1$$

My questions are to find expressions for

1. Second largest eigenvalue modulus (SLEM) or eigenvalues

2. Inverse of $X$

Thanks

Answer 1

$1-\sqrt{2} c$ is the second largest eigenvalue. For $c=-\frac{1}{2}$ it gives the second largest modulus. According to maple this is the determinant: $$ (2d-1)(-1+2c^2)(2c^4-4c^2+2dc^2-4d^3+2d-2d^2+1) . $$