Proof : Suppose first that k is coprime to m. If S has period k,then { a_1= a_(k+1)=⋯= a_(mk+1).Now reduce the indices modulo m noting that this produces a complete set of residues modulo m. This implies that all of the terms of S are the same, and hence the minimum period is 1,which is a contradiction.So S does not have period k coprime to m.