# Thread: Spectrum - proof of the theorem

1. ## Spectrum - proof of the theorem

Hi,

Where I can find (in which book) proof of the following theorem:

"The spectrum of a function in the algebra is the set of values of the function"

or how can I proof it?

Thanks for any help.

2. Originally Posted by Arczi1984
Hi,

Where I can find (in which book) proof of the following theorem:

"The spectrum of a function in the algebra is the set of values of the function"

or how can I proof it?
If C(S) is the Banach algebra of continuous functions on a compact set S, and $f\in C(S)$, the inverse of $f-\lambda1$ will be the function g given by g(t) = $\frac1{f(t)-\lambda}$ (provided that such a function exists). If f(t) is never equal to $\lambda$ then g will be defined on all of S, bounded (because S is compact) and continuous. Therefore $f-\lambda1$ has an inverse in C(S) and hence $\lambda\notin\sigma(f)$. But if there is a value of t for which $f(t)=\lambda$ then g is not defined at that value of t. In that case $f-\lambda1$ has no inverse in C(S) and hence $\lambda\in\sigma(f)$.