On Normal Criterion Of A Family Of Meromorphic Functions

In this paper we derive the following result:Let F be a family of meromorphic functions on a domain D, all of whose zeros and poles have multiplicity at least2,f(0)≠0, and let a(z)(≠0) be a meromorphic function which has no common poles with f(z). Suppose that for each f∈F, f(z)=a(z)(?)f’(z)=z, then F is normal on D.