The essential spectrum of T, usually denoted σess(T), is the set of all complex numbers λ such that
is not a Fredholm operator.
Here, an operator is Fredholm if its range is closed and its kernel and cokernel are finite-dimensional. Furthermore, I denotes the identity operator on X, so that I(x) = x for all x in X.
Read more about Essential Spectrum: The Essential Spectrum of General Bounded Operators
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