GUENTER LUMER is a Professor in the Department of Mathematics at the University of Mons-Hainaut, Mons. Belgium, and a member/researcher of the Solvay Institute for Physics and Chemistry, Brussels, Belgium. He is also President of the Contact Group on Partial Differential Equations of the Belgian National Fund for Scientific Research. The coeditor of Erolution Equations, Control Theory, and Biomathematics (Marcel Dekker, Inc.) and author or coauthor of over JOO professional papers and monographs. he is a member of the American Mathematical Society, the Belgian Mathematical Society, and the French Mathematical Society. Dr. Lumer received the M.Sc. degree ( 1957) in electrical engineering from the University of Montevideo, Uruguay, and the Ph.D. degree ( 1959) in mathematics from the University of Chicago, Illinois. LUTZ WEIS is Professor of Mathematics at the University of Karlsruhe, Germany. The author or coauthor of numerous professional papers on asymptotics, semigroups, spectral theory, and other topics, Dr. Weis is a member of the American Mathematical Society and the German Mathematical Society. He received the Ph.D. degree (1974) in mathematics from the University of Bonn, Germany.