# Gelfand's Formula. $r(T)=\lim_{n \to\infty}\sqrt[n]{\|T^{n}\|}$

Can you indicate me a material where I cand find the proof of Gelfand's Formula. I heard that there is a proof with polynomials.

Gelfand's Formula :

If $T \in B(X)$ then: $$r(T)=\lim_{n \to\infty}\sqrt[n]{\|T^{n}\|}$$

-