$\begin{bmatrix}
 1 & 0 & 0 & 1 \\
 0 & 1 & 1 & 0 \\
 0 & 1 & 1 & 0 \\
 1 & 0 & 0 & 1 \\
 \end{bmatrix}$

tried to find Eigen value for this matrix 
$\begin{bmatrix}
 1-\lambda & 0 & 0 & 1 \\
 0 & 1-\lambda & 1 & 0 \\
 0 & 1 & 1-\lambda & 0 \\
 1 & 0 & 0 & 1-\lambda \\
 \end{bmatrix}$
and got $\lambda^4-4\lambda^3+6\lambda^2-4\lambda$   

so $\lambda(\lambda^3-4\lambda^2+6\lambda-4)$   

but I tried to factorize $\lambda^3-4\lambda^2+6\lambda-4$ got $(\lambda-2)<br>\lambda^2-2\lambda+2$ and cannot factorize $\lambda^2-2\lambda+2$ .

 is my calculation wrong? 
thanks!!