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From Wikipedia, the free encyclopedia
In mathematics, or more specifically in spectral theory, the Riesz projector is the projector onto the eigenspace corresponding to a particular eigenvalue of an operator (or, more generally, a projector onto an invariant subspace corresponding to an isolated part of the spectrum). It was introduced by Frigyes Riesz in 1912.[1][2]
Let be a closed linear operator in the Banach space . Let be a simple or composite rectifiable contour, which encloses some region and lies entirely within the resolvent set () of the operator . Assuming that the contour has a positive orientation with respect to the region , the Riesz projector corresponding to is defined by
here is the identity operator in .
If is the only point of the spectrum of in , then is denoted by .
The operator is a projector which commutes with , and hence in the decomposition
both terms and are invariant subspaces of the operator . Moreover,
If and are two different contours having the properties indicated above, and the regions and have no points in common, then the projectors corresponding to them are mutually orthogonal:
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