The Tensor Product of Semigroups and the Operator Equation SX - XT = A J. Freeman Indiana University Mathematics Journal 1970 text pdf 10.1512/iumj.1970.19.19065 10.1512/iumj.1970.19.19065 en Indiana Univ. Math. J. 19 (1970) 819 - 828 state-of-the-art mathematics http://iumj.org/access/