Denote by S a complex
(nondegenerate) Banach space. Suppose that T is a transformation from a subset of
S to S. A complex number λ is said to be in the resolvent of T if (λI −T)−1 exists,
has domain S and is Fréchet differentiable (i.e., if p is in ∕S there is a
unique continuous linear transformation F = [(λI − T)−1]′(p) from S to S so
that
and locally Lipschitzean everywhere on S. A complex number is said to be in the
spectrum of T if it is not in the resolvent of T.
Suppose in addition that the domain of T contains an open subset of S on which
T is Lipschitzean.
Theorem. T has a (nonempty) spectrum.
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