Publicationes Mathematicae Debrecen appears quarterly and publishes original research papers on pure mathematical topics. It welcomes contributed papers that develop interesting, or important, new mathematical ideas and results or solve outstanding problems. All papers are refereed for correctness and suitability for publication. Publicationes Mathematicae Debrecen is covered by the Mathematical Reviews, the Zentralblatt der Mathematik, the Science Abstracts and the Science Citation Index. Published quarterly by the Institute of Mathematics, University of Debrecen H-4002 Debrecen, P O Box 400, Hungary. ISSN 0033 - 3883