Riesz projection

For a spectral subset isolated by a contour $\Gamma$ in the resolvent set,projects onto its spectral subspace. Norm-small perturbations preserve the rank of this projection.

For a bounded operator on a complex Banach space, partition a disconnected spectrum into two nonempty compact pieces. The holomorphic function equal to one near the first piece and zero near the second defines a Riesz projection. The holomorphic spectral mapping theorem makes it neither zero nor identity. Its closed range is a proper nonzero invariant vector subspace. This need not be an orthogonal projection, and the argument does not require a normal operator.