Normal families of meromorphic functions concerning shared values

In this paper we study the problem of normal families of meromorphic functions concerning shared values and prove that a family F of meromorphic functions in a domain D is normal if for each pair of functions f and g in F, f′−afn and g′−agn share a value b in D where n is a positive integer and a,b are two finite constants such that n⩾4 and a≠0. This result is not true when n⩽3.