1. On the roots of spectral operators;Apostol,1968
2. On the roots of spectral operator-valued analytic functions;Apostol;Rev. Roum. Math. Pures et Appl.,1968
3. Spectral decompositions and functional calculus;Apostol;Rev. Roum. Math. Pures et Appl.,1968
4. Algebraic functions of normal operators;Foguel;Israel J. Math.,1968