Let $\left( U _ { n } \right) _ { n \in \mathbb { N } }$ be the sequence of polynomials defined by $U _ { 0 } = 1 , U _ { 1 } = X - 1$ and $\forall n \in \mathbb { N } , U _ { n + 2 } = ( X - 2 ) U _ { n + 1 } - U _ { n }$. For all $n \in \mathbb { N }$, show that $U _ { n }$ is monic of degree $n$, and determine the value of $U _ { n } ( 0 )$.