The minimal operator and the maximal operator of an elliptic pseudodifferential operator with symbols on Z n × T n are proved to coincide and the domain is given in terms of a Sobolev space. Ellipticity and Fredholmness are proved to be equivalent for pseudo-differential operators on Z n . The index of an elliptic pseudo-differential operator on Z n is also computed.2010 Mathematics Subject Classification. Primary 35S05, 47G30; Secondary 43A85, 43A77. Key words and phrases. pseudo-differential operators, minimal and maximal operators, ellipticity, Fredholmness, index.Proof. of Theorem 3.5 Let ǫ > 0, be such that t − s − ǫ > 0.Since J −1 ǫ J −s is a discrete pseudo-differential operator of order s + ǫ , it follows that the composition J ǫ iJ −1 ǫ J −s of the mappings