@incollection{PopPur:88,
  publisher = {Babe\c s-Bolyai University},
  title = {A generalization of the Zupnik's theorem relatively to
           $n$-semigroups},
  olume = {Preprint no. 5},
  author = {Pop, M. S. and Purdea, Ioan},
  year = 1988,
  pages = {56--62},
  series = {Faculty of Mathematics Research Seminaries},
}