Document Type: Regular Paper
School of Mathematics, Statistics and Computer Science,
College of Sciences, University of Tehran, P.O. Box 14155-6455, Teheran, Iran
Department of Mathematics, University of Mazandaran, Babolsar, Iran
Department of Mathematics, Babol University of Technology, Babol, Iran
We consider the fundamental relation on a semihypergroup to interpret the lower and upper approximations as subsets of the fundamental semigroup and we give some results in this connection. Also, we introduce the notion of a bi-hyperideal to study the relationship between approximations and bi-hyperideals.