Weak Spectral Inverses which are Partial Isometries
George D. Poole, Thomas L. Boullion · SIAM Journal on Applied Mathematics · 1972
If A is a square partial isometry, necessary and sufficient conditions are given for the class $W( A )$ of left weak spectral inverses to contain a partial isometry. Also necessary and sufficient conditions are given for a nilpotent, idempotent and tripotent matrix to be a partial isometry.