Hyperinvariant Subspaces of the Direct Sum of Certain Contractions Pei Wu Indiana University Mathematics Journal 1978 text pdf 10.1512/iumj.1978.27.27019 10.1512/iumj.1978.27.27019 en Indiana Univ. Math. J. 27 (1978) 267 - 274 state-of-the-art mathematics http://iumj.org/access/