Ascent, Descent, Nullity and Defect of Products of Operators J. BuoniJ. Faires Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25054 10.1512/iumj.1976.25.25054 en Indiana Univ. Math. J. 25 (1976) 703 - 707 state-of-the-art mathematics http://iumj.org/access/