1. C. Badea and M. Mbekhta, Operators similar to partial isometries, Acta Sci. Math. (Szeged) 71 (2005), 663–680.
2. S. L. Campbell, Linear operators for which $$T^*T$$ T ∗ T and $$TT^*$$ T T ∗ commute, Pacific J. Math. 34 (1972), 177–180.
3. B. P. Duggal, C. S. Kubrusley, and N. Levan, Contractions of class Q and invariant subspaces, Bull. Korean Math. Soc. 42 (2005), 169–177.
4. Ivan Erdelyi, Partial isometries closed under multiplication on Hilbert spaces, J. Math. Anal. Appl. 22 (1968), 546–551.
5. T. Furuta and R. Nakamoto, Some theorems on certain contraction operators, Proc. Japan Acad. Ser. A Math. Sci. 45 (1969), 565–567.