 Solution

$\sigma (T)=\{\lambda :T-\lambda I\text{ is not invertible}\}$ and $r(T)={\sup }_{\lambda \in \sigma (T)}|\lambda |$, well-defined because the spectrum is nonempty compact. If $|\lambda |>\Vert T^n{\Vert }^{1/n}$, factor $T^n-{\lambda }^{n}I$ and use a Neumann series; spectral mapping then excludes $\lambda$. Thus $r(T)\le {\inf }_n\Vert T^n{\Vert }^{1/n}$.