A normal criterion concerning zero numbers

Abstract Let $$n \ge 4$$ n ≥ 4 be a positive integer, $$\mathcal {F}$$ F family of meromorphic functions in D and let $$a(z)(\not \equiv 0), b(z)$$ a ( z ) ≢ 0 , b two holomorphic . If, for any function $$f \in \mathcal { F}$$ f ∈ , (1) $$f(z) \ne \infty $$ ≠ ∞ when $$a(z)=0$$ = (2) $$f'(z)-a(z)f^{n}(z)-b(z)$$ ′ - has at most one zero then is normal