\(\small |A {-\lambda}E| \, := \, \left|\begin{array}{rrr}-\ell + 2&-1&1\\-1&-\ell + 2&-1\\-1&1&-\ell\\\end{array}\right|=0\)
===> \(-\left(\ell - 1 \right)^{2} \; \left(\ell - 2 \right) = 0\)
===>\(\small \left(\begin{array}{rrrr}\lambda=&1&\left(\begin{array}{rrr}1&-1&1\\-1&1&-1\\-1&1&-1\\\end{array}\right)&\left(\begin{array}{r}x1\\x2\\x3\\\end{array}\right) = 0\\\lambda=&2&\left(\begin{array}{rrr}0&-1&1\\-1&0&-1\\-1&1&-2\\\end{array}\right)&\left(\begin{array}{r}x1\\x2\\x3\\\end{array}\right) = 0\\\end{array}\right)\)
==> \(DimEigenraum \, := \, \left\{ 2, 1 \right\} \)
\(\small ER_{λ_{1},λ_{2}} \, := \, \left(\begin{array}{rrr}1&-1&-1\\1&0&1\\0&1&1\\\end{array}\right)\)