A Logarithmic Derivative Lemma in Several Complex Variables and Its Applications
TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 363, Number 12, December 2011, Pages 6257–6267 S 0002-9947(2011)05226-8 Article electronically published on June 27, 2011 A LOGARITHMIC DERIVATIVE LEMMA IN SEVERAL COMPLEX VARIABLES AND ITS APPLICATIONS BAO QIN LI Abstract. We give a logarithmic derivative lemma in several complex vari- ables and its applications to meromorphic solutions of partial differential equa- tions. 1. Introduction The logarithmic derivative lemma of Nevanlinna is an important tool in the value distribution theory of meromorphic functions and its applications. It has two main forms (see [13, 1.3.3 and 4.2.1], [9, p. 115], [4, p. 36], etc.): Estimate (1) and its consequence, Estimate (2) below. Theorem A. Let f be a non-zero meromorphic function in |z| <R≤ +∞ in the complex plane with f(0) =0 , ∞. Then for 0 <r<ρ<R, 1 2π f (k)(reiθ) log+ | |dθ 2π f(reiθ) (1) 0 1 1 1 ≤ c{log+ T (ρ, f) + log+ log+ +log+ ρ +log+ +log+ +1}, |f(0)| ρ − r r where k is a positive integer and c is a positive constant depending only on k. Estimate (1) with k = 1 was originally due to Nevanlinna, which easily implies the following version of the lemma with exceptional intervals of r for meromorphic functions in the plane: 1 2π f (reiθ) (2) + | | { } log iθ dθ = O log(rT(r, f )) , 2π 0 f(re ) for all r outside a countable union of intervals of finite Lebesgue measure, by using the Borel lemma in a standard way (see e.g.
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