k_A \left( \lambda \right) = \left| {A - \lambda I} \right| = \left| {\begin{array}{*{20}c}
 {1 - \lambda } & 2 & 3 \\
 0 & {1 - \lambda } & 2 \\
 0 & 0 & {2 - \lambda } \\

 \end{array} } \right| = \left( {2 - \lambda } \right)\left( {1 - \lambda } \right)^2 \Rightarrow \sigma \left( A \right) = \left\{ {1,2} \right\}