Positive derivatives and increasing functions.
Let ℱ be a family of meromorphic functions on a plane domain D, all of whose zeros are of multiplicity at least k ≥ 2. Let a, b, c, and d be complex numbers such that d ≠ b,0 and c ≠ a. If, for each f ∈ ℱ, , and , then ℱ is a normal family on D. The same result holds for k=1 so long as b≠(m+1)d, m=1,2,....
Page 1