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A quasinormal criterion of meromorphic functions and its application
Journal of Inequalities and Applications volume 2014, Article number: 389 (2014)
Abstract
In this paper, we obtain a quasinormal criterion of meromorphic functions and give an example of an application in the value distribution theory. More than anything, we provide a general method to solve some problems in the value distribution theory.
MSC:30D35, 30D45.
1 Introduction
We use the following notation. Let ℕ denote the set of positive integers. Let ℂ be complex plane and D be a domain in ℂ. For and , , and . Let denote the number of poles of in D (counting multiplicities) and denote the number of poles of in D (not counting multiplicities). We write in D to indicate that the sequence converges to f in the spherical metric uniformly on compact subsets of D and in D if the convergence is in the Euclidean metric. For f meromorphic in D, set
The Ahlfors-Shimizu characteristic is defined by . Let denote the usual Nevanlinna characteristic function. Since is bounded as a function of r, we can replace with in this paper.
Recall that an elliptic function [1] is a meromorphic function h defined in ℂ for which there exist two nonzero complex numbers and with not real such that for all z in ℂ.
Recall that a family ℱ of functions meromorphic in D is said to be quasinormal in D [2] if from each sequence one can extract a subsequence which converges locally uniformly with respect to the spherical metric in , where the set E (which may depend on ) has no accumulation points in D. If E can always be chosen to satisfy , ℱ is said to quasinormal of order ν in D. Thus a family is quasinormal of order 0 in D if and only if it is normal in D. The family ℱ is said to be (quasi)normal at if it is (quasi)normal in some neighborhood of . Thus ℱ is quasinormal in D if and only if it is quasinormal at each point . On the other hand, ℱ fails to be quasinormal of order ν in D precisely when there exist points in D and a sequence such that no subsequence of is normal at ().
In 2007, Nevo et al. proved the following quasinormal criterion.
Theorem A [[3], Theorem 1]
Let ℱ be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . If there exists a holomorphic function H univalent in D such that for all and all , then ℱ is quasinormal of order 1 in D.
For , we extend Theorem A in this paper. In fact, we obtain the following result.
Theorem 1.1 Let be an integer, and let ℱ be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . Let H be a nonconstant meromorphic function. Suppose that there exists such that for each , . If for all and all , then ℱ is quasinormal of order ν in D.
Remark 1.1 For , Theorem A is the special case of Theorem 1.1 with and for all . Unfortunately, the restricted condition that largely restricts the applications of Theorem A, so it is important to remove the condition’s restriction.
For convenience, we give a generalized form of Theorem 1.1.
Proposition 1.2 Let be an integer, and let be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . Let H be a nonconstant meromorphic function, and there exists such that for each , . Let be a family of meromorphic functions in D such that and have the same zeros and poles with the same multiplicity, and in D. If for all and all , then is quasinormal of order ν in D.
Moreover, for each subsequence of , there exist a subsequence of (still denoted by ) and a corresponding point set E which has no accumulation points in D such that:
-
(1)
in , where is meromorphic or identically infinite there;
-
(2)
for each , and no subsequence of is normal at ;
-
(3)
for each , there exist and such that for sufficiently large k, , where and only depend on ; and
-
(4)
for each , in .
The value distribution theory of meromorphic functions occupies one of the central places in complex analysis which now has been applied to complex dynamics, complex differential and functional equations, Diophantine equations, and others.
In his excellent paper [4], Hayman studied the value distribution of certain meromorphic functions and their derivatives under various conditions. Among other important results, he proved that if is a transcendental meromorphic function in the plane, then either assumes every finite value infinitely often, or every derivative of assumes every finite nonzero value infinitely often. This result is known as Hayman’s alternative. Thereafter, the value distribution of derivatives of transcendental functions continued to be studied.
In 1995, Bergweiler and Eremenko proved the following result.
Theorem B [[5], Theorem 3]
Let f be a meromorphic function of finite order in ℂ. If f has infinitely many multiple zeros, then assumes every finite nonzero value infinitely often.
In 2008, Pang et al. obtained the following result.
Theorem C [[6], Theorem 1]
Let f be a transcendental meromorphic function in ℂ, all but finitely many of whose zeros are multiple, and let R (≢0) be a rational function. Then has infinitely many zeros.
R is a small function compared with f in Theorem C. Specifically, as in Theorem C. A natural problem arises: What can we say if the rational function R in Theorem C is replaced by a more general small function ? In this direction, we obtain the following result.
Theorem 1.3 Let be an integer, let be a meromorphic function in ℂ, and let (≢0), where is an elliptic function and is a rational function. Suppose that all but finitely many zeros of f have multiplicity at least and as . Then the equation has infinitely many solutions (including the possibility of infinitely many common poles of and ).
2 Preliminary lemmas
Lemma 2.1 [[7], Corollary 2]
If is a nonconstant elliptic function with primitive periods , , where is not real, then as , where is a constant.
Lemma 2.2 Let ℱ be a family of functions meromorphic in D, all of whose zeros have multiplicity at least k, and suppose that there exists such that whenever . Then if ℱ is not normal at , there exist, for each ,
-
(a)
points , ;
-
(b)
functions ; and
-
(c)
positive numbers
such that in ℂ, where g is a nonconstant meromorphic function in ℂ such that . In particular, g has order at most 2.
This is the local version of [[8], Lemma 2] (cf. [[9], Lemma 1]; [[10], pp.216-217]). The proof consists of a simple change of variable in the result cited from [8]; cf. [[11], pp.299-300].
Lemma 2.3 [[12], Lemma 3.6]
Let be a family of functions meromorphic in . Suppose that in , where f is a nonconstant meromorphic function or in . If there exists such that for each n, , then there exists such that .
Lemma 2.4 [[3], Lemma 3]
Let k be a positive integer, let be a family of meromorphic functions in D, and let be a family of holomorphic functions in D such that , where in D. If for all and all , and , then is normal in D.
Lemma 2.5 Let be a sequence in D which has no accumulation points in D, and let be a family of holomorphic functions in D such that in D, where in D. Let be a family of meromorphic functions in D, all of whose zeros have multiplicity at least , such that for all and all . Suppose that:
-
(a)
no subsequence of is normal at ;
-
(b)
in .
Then
-
(c)
there exists such that has a single (multiple) zero in for sufficiently large n;
-
(d)
there exists such that for each , has a single simple pole in for sufficiently large n; and
-
(e)
for . Equivalently, f extends to an analytic function in such that and for .
Remark 2.1 Since Lemma 2.5 is not stated explicitly in [3], let us indicate how it follows from the results of that paper. Suppose first that (c) has been shown to hold. By Lemma 7 in [3], (d) and (e) hold. Next, suppose that (c) do not hold. Similar to the treatment in Case 2 (pp.13-16) of the proof of Theorem 1 in [3], we can finally derive a contradiction.
Lemma 2.6 Let k be a positive integer, and let be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . Let be a family of meromorphic functions in D such that in D, where H is a holomorphic univalent function in D. If for all and all , then is quasinormal of order 1 in D.
Lemma 2.6 can be proved by an exactly analogous argument as in the proof of Theorem 1 in [3]. In fact, there is no essential difference between Lemma 2.6 and Theorem 1 in [3], so we do not give the proof of Lemma 2.6.
Lemma 2.7 [[13], Lemma 12]
Let R be a nonconstant rational function satisfying in ℂ. Then either or , where and .
Lemma 2.8 [[14], Theorem 1]
Let k be a positive integer, let f be a transcendental meromorphic function in ℂ, and let R (≢0) be a rational function. If all but finitely many zeros of f have multiplicity at least , then has infinitely many zeros.
Remark 2.2 The proof of Lemma 2.8 is based, quite naturally, on a combination of ideas from [15] and [6]. In fact, fully understanding the ideas and methods in [15] and [6], one can give the proof of Lemma 2.8 without difficulty.
Lemma 2.9 [[16], Lemma 6]
Let k and l be positive integers, and let be a rational function, all of whose zeros have multiplicity at least k. If in ℂ, then is a constant.
Lemma 2.10 [[16], Lemma 10]
Let k, l be positive integers with , let be a family of holomorphic functions, and let be a family of meromorphic functions, all of whose poles are multiple and all of whose zeros have multiplicity at least . Suppose that:
-
(1)
and are defined in , where the positive sequence increases to ∞;
-
(2)
in ℂ;
-
(3)
for ;
-
(4)
in ℂ; and
-
(5)
.
Then f has a zero in Δ.
Lemma 2.11 Let f be a nonconstant meromorphic function of finite order in ℂ, all of whose zeros have multiplicity at least . If in ℂ, then
for some a and b (≠a) in ℂ.
This follows from results in [17], specifically Lemma 6 (whose proof depends in an essential fashion on Corollary 3 of [5]) and Lemma 8. As an immediate consequence of Lemma 2.11, we have the following result, which appears as Lemma 9 of [17].
Lemma 2.12 [[17], Lemma 9]
Let k be a positive integer, and let f be a meromorphic function of finite order in ℂ, all of whose poles are multiple and whose zeros all have multiplicity at least . If for some constant and all , then is a constant.
3 Auxiliary lemmas
Lemma 3.1 Let and be families of meromorphic functions in D, and let and be meromorphic functions in D. Suppose that:
-
(a)
and in D, and
-
(b)
in D.
Then, either or in D.
Proof Suppose that in D. Set . By (a) and (b), we have
Since is holomorphic in D and A has no accumulation points in D, we have
Thus is a holomorphic function in D and then in D.
In order to show that in D, we need only show that f and ψ have no common poles in D. Otherwise, we assume that is a pole of order of f and a pole of order of ψ. Set . Obviously, is a zero of of order at most m.
By (a) and Hurwitz’ theorem, there exists such that and for each , and have at least and (counting multiplicities) poles respectively in for sufficiently large n. By (b), and have no common poles in , and hence has at least poles (counting multiplicities) in . Since δ can be made arbitrarily small, is a zero of of order at least by (3.1). Thus . This is a contradiction. □
Lemma 3.2 Let k be a positive integer, and let be a family of meromorphic functions in Δ, all of whose zeros have multiplicity at least . Let be a sequence of meromorphic functions in Δ such that in Δ, where b (≢0) is a meromorphic function and . Suppose that:
-
(a)
b and have the same zeros and poles with the same multiplicity;
-
(b)
for all and all , ;
-
(c)
there exists points in Δ such that and ; and
-
(d)
in , where is a meromorphic function in .
Then in .
Proof Set . By (a) and (b), , and hence . Since all zeros of have multiplicity at least , we have . Hence and for sufficiently large n. Since and for sufficiently large n, is not equicontinuous at 0, and hence is not normal at 0.
By Lemma 3.1, we have either or in . Suppose that in . By the assumptions, there exists such that has no poles on and has no zeros on . Thus, we have
By the maximum principle, (3.2) holds in , and then is normal at 0. This is a contradiction. Thus in . □
Lemma 3.3 Let k be a positive integer, let be a family of meromorphic functions in D, and let be a family of meromorphic functions in D such that in D, where . If all and all , and , then is normal in D.
Proof By Lemma 2.4, it suffices to prove that is normal at points which h has poles or zeros. Without loss of generality, we assume that , , where in Δ and l (≠0) is an integer. Then is normal in .
Suppose that is not normal at 0. Taking a subsequence and renumbering, we may assume that no subsequence of is normal at 0. Since in Δ, there exists such that and in . By the argument principle, we have, for sufficiently large n,
Since , we have . Thus and has poles (otherwise in ) which are different from the poles of . Hence . This is a contradiction. □
Lemma 3.4 Let k be a positive integer, and let be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . Let be a family of meromorphic functions in D such that , where in D. If for all and all , then is quasinormal in D.
Proof It suffices to show that is quasinormal in a neighborhood of each point of D. Let . There exists such that and ψ is holomorphic and does not vanish in . By Lemma 2.6, is quasinormal in .
Suppose now that is not quasinormal at p. Then there exist points () and a subsequence of (still denoted by ) such that and no subsequence of is normal at any , . Set . Taking a subsequence of (still denoted by ), we may assume that in . By Lemma 2.5, we have and . It follows that H is holomorphic in and there. Moreover, since ψ has no essential singularity at p, the same is true of H. But that for implies that , and hence , which contradicts . □
Lemma 3.5 Let k and l be positive integers, and let R be a rational function. If for all , then
where n () is an integer and ().
Proof Obviously, . Then is a nonconstant rational function. By Lemma 2.7,
where n (≥k) is an integer and . In fact, if , then is a pole of of order at least k, and hence . Now, we have
where is a polynomial of degree k, is a polynomial of degree at most , and is a constant. Thus has the following form:
where n () is an integer and (). □
Lemma 3.6 Let d and k (≥1) be integers, and let f be a transcendental meromorphic function, all of whose zeros have multiplicity at least . Set with if . If , then there exist points and positive numbers such that
Proof By standard results in Nevanlinna theory, and as . Thus, , and then .
We claim that there exist and such that
Otherwise there would exist and such that for all . From this follows
Obviously, . This contradicts the fact that .
By (3.4), there exist points such that and . Set . Clearly, , and hence is not normal at 0. Obviously, all zeros of have multiplicity at least in Δ for sufficiently large n. Using Lemma 2.2 for , there exist points , positive numbers , and a subsequence of (still denoted by ) such that in ℂ, where G is a nonconstant meromorphic function in ℂ, all of whose zeros have multiplicity at least .
Since is not a constant (otherwise, either is a constant, or the zero of have multiplicity at most k), we may assume is not a zero or pole of . Set . Now we have
where . Obviously, , , and for .
Now, we have and
where () are constants and . Set . Obviously, and , and hence . □
Lemma 3.7 Let k be a positive integer, and let be a family of meromorphic functions in D, all of whose poles are multiple and whose zeros all have multiplicity at least . Let be a family of meromorphic functions in D such that in D, where h (≠0) is a holomorphic function in D. If for each and , then is normal in D.
Proof Suppose that is not normal at a point . Then by Lemma 2.2, there exist points , positive numbers and a subsequence of (still denoted by ) such that in ℂ, where g is a nonconstant meromorphic function in ℂ, all of whose poles are multiple and whose zeros all have multiplicity at least . In particular, g has order at most 2. Obviously, and (≠0) in ℂ. By Lemma 3.1, we have either or in ℂ. Firstly, suppose that in ℂ. It follows that . We arrive at a contradiction as is nonconstant and all zeros of have multiplicity at least . Secondly, suppose that in ℂ. By Lemma 2.12, then is a constant. A contradiction. □
Lemma 3.8 Let k be a positive integer, and let be a family of meromorphic functions in D, all of whose poles are multiple and whose zeros all have multiplicity at least . Let be a family of meromorphic functions in D such that in D, where h (≠0) is a meromorphic function in D. Suppose that h and have the same poles, all with the same multiplicity. If for all and all , then is normal in D.
Lemma 3.8 can be proved by an exactly analogous argument as in the proof of Theorem 1 in [16]. To facilitate the reading, Lemma 3.8 was proved in this paper.
Lemma 3.9 Let be an integer, and let be a family of meromorphic functions in D, all of whose zeros have multiplicity at least . Let be a family of meromorphic functions in D such that in D, where h () is a meromorphic function. Let be a set which has no accumulation points in D. Suppose that:
-
(∗a)
h and have the same zeros and poles with the same multiplicity;
-
(∗b)
for all and all , ;
-
(∗c)
for each , no subsequence of is normal at ; and
-
(∗d)
in , where is meromorphic or identically infinite there.
Then
-
(∗e)
for each , ;
-
(∗f)
for each , there exist and such that for sufficiently large n, , where and only depend on ; and
-
(∗g)
for each , in .
4 Proof of Lemma 3.8
Proof Since normality is a local property, by Lemma 3.7, we only need to prove that is normal at every pole of . Making standard normalizations, we may assume and
where l is a positive integer, , and for all .
Set . Since h and have the same poles, all with the same multiplicity and in D, we have in Δ.
Clearly, it is enough to show that is normal at . Suppose, on the contrary, that is not normal at 0. By Lemma 3.7, is normal in . Taking a subsequence and renumbering, we may assume that no subsequence of is normal at 0. Our goal is to obtain a contradiction in the sequel.
We distinguish two cases.
Case 1. .
By Lemma 2.2, there exist points , positive numbers and a subsequence of (still denoted by ) such that
where is a nonconstant meromorphic function in ℂ. By Hurwitz’s theorem, all poles of are multiple and all zeros of have multiplicity at least .
Taking a subsequence and renumbering, we may assume as , where or . Again we distinguish two subcases.
Subcase 1.1. .
Set . Obviously, all poles of are multiple and all zeros of have multiplicity at least ,
Then, by Lemma 3.7, the family is normal in Δ. Taking a subsequence and renumbering, we may assume that in Δ. Obviously, all zeros of have multiplicity at least in Δ.
We claim that is a meromorphic function in Δ. Otherwise, suppose that in Δ. Then
Thus, in ℂ. A contradiction.
We claim that . Since is a nonconstant meromorphic function in ℂ, there exist such that . Noting that as , we have
For any , we have
This implies that in and thus is a constant in ℂ. It follows that . We arrive at a contradiction as is nonconstant and all zeros of have multiplicity at least in ℂ.
Subcase 1.2. ().
Clearly, we have
By Lemma 3.1, either or in ℂ. The latter possibility contradicts the fact that all poles of have multiplicity at least (>l). Thus in ℂ. It follows from Lemma 2.8 and Lemma 2.9 that is a constant. A contradiction.
Case 2. .
Set
Clearly, all poles of are multiple and all zeros of have multiplicity at least in Δ. Since , we have . Thus, for each n, . Obviously, has a zero of order at least l at for each n.
We first prove that is normal in Δ. Suppose that is not normal at . Then by Lemma 2.2, there exist points , positive numbers , and a subsequence of (still denoted by ) such that
where is a nonconstant meromorphic function in ℂ. In particular, has order at most 2. By Hurwitz’s theorem, all poles of are multiple and all zeros of have multiplicity at least .
Taking a subsequence and renumbering, we may assume as , where or . Again we distinguish two subcases.
Subcase 2.1. .
By simple calculation, we have
Then we have
On the other hand, we have and then for ,
Since , is locally bounded in . Thus,
Since , Hurwitz’s theorem yields that either or in ℂ. If in ℂ, then by Lemma 2.12, is a constant. A contradiction. Thus , and then . This contradicts the fact that all zeros of have multiplicity at least .
Subcase 2.2. ().
Clearly, all poles of are multiple and all zeros of have multiplicity at least , and has a zero of order at least l at .
Set . Then, we have
Noting that in ℂ, we have
Since has a zero at , by (4.1), there exists such that is holomorphic in for sufficiently large n, and thus is holomorphic in for sufficiently large n. By the maximum principle, we have
Obviously, all poles of are multiple and all zeros of have multiplicity at least . Since has a zero of order at least l at , we have and thus is a meromorphic function in ℂ. Noting that
By Lemma 3.1, either or in ℂ. The latter possibility contradicts the fact that . Thus in ℂ. It follows from Lemma 2.8 and Lemma 2.9 that is a constant. Next we will show that this is impossible. Indeed, suppose that . Since G is not a constant, . Then we have
Suppose first that there exists such that in for sufficiently large n. By Lemma 3.3, is normal in . But this contradicts our assumption that no subsequence of is normal at 0. Hence, taking a subsequence and renumbering, we may assume that is the zero of of smallest modulus and . Since (≠0), we have . Set
In view of the fact that
and in Δ, by Lemma 3.3, is normal in Δ. By Lemma 3.7, is normal in . Hence, is normal in ℂ. Therefore, there exists a subsequence (still denoted by ) such that in ℂ. By the definition of , we get and thus . Since
we get that . Thus is nonconstant. However, since and in Δ, we have in Δ by Hurwitz’s theorem. However, Lemma 2.10 implies that has a zero in Δ. A contradiction.
Thus is normal in Δ. It remains to show that is normal at 0. Since is normal in Δ, then is equicontinuous in Δ with respect to the spherical distance. On the other hand, for each n, so there exists such that for all n in . It follows that is holomorphic in for all n. Since is normal in , there exists a subsequence of (still denoted by ) which converges locally uniformly in . The maximum modulus principle implies that converges locally uniformly in , and thus normal at , which contradicts our assumption that no subsequence of is normal at 0. □
5 Proof of Lemma 3.9
Proof It suffices to prove that each subsequence of has a subsequence which satisfies that (∗f), and prove that (∗e) and (∗g) hold. So suppose that we have a subsequence of , which (to avoid complication in notation) we again call .
Without loss of generality, for each , we may assume that , , , and
in Δ, where and in Δ.
We consider the following three cases.
Case 1. .
We will derive a contradiction in the case, and hence (∗e) holds. For convenience, we set , and then
in Δ, where in Δ, , and m is a positive integer. Clearly, we have in , in , and .
Subcase 1.1. For sufficiently large n, .
We claim that for each , there exists at least one zero of in for sufficiently large n. Otherwise, there exist (>0) and a subsequence of (still denoted by ) such that in . By Lemma 3.3, is normal at 0. This contradicts (∗c).
Taking a subsequence and renumbering, we may assume that (≠0) is the zero of of smallest modulus and . Set . We have:
-
(A1)
in Δ;
-
(A2)
all zeros of have multiplicity at least and ;
-
(A3)
and in ℂ.
By Lemma 3.3 and Lemma 3.4, is normal in Δ and quasinormal in ℂ. Thus, there exist a subsequence of (still denoted by ) and such that:
-
(B1)
has no accumulation points in ℂ;
-
(B2)
for each , no subsequence of is normal at ;
-
(B3)
in .
Obviously, and all zeros of have multiplicity at least in .
Subcase 1.1.1. .
Obviously, by (A2). Let . By Lemma 2.5, , which contradicts . Thus is an empty set. Since , is a meromorphic function in ℂ. By Lemma 3.1 and (A3), either or in ℂ. If , then , which contradicts . If , then by Lemma 2.8 and Lemma 2.9, F is a constant function. Since , in ℂ. Now,
We claim that for each , there exists at least one pole of in for sufficiently large n. Otherwise, there exist (>0) and a subsequence of (still denoted by ) such that has no poles in . Since and , we have . Thus in . By Lemma 3.8, is normal at 0. This contradicts (∗c).
Taking a subsequence and renumbering, we may assume that (≠0) is the pole of of smallest modulus and . By Hurwitz’s theorem and (5.1), . Set , we have:
-
(C1)
is holomorphic in Δ;
-
(C2)
;
-
(C3)
all zeros of have multiplicity at least ;
-
(C4)
and in ℂ.
By Lemma 3.8 and Lemma 3.4, is normal in Δ and quasinormal in ℂ. Thus, there exist a subsequence of (still denoted by ) and such that:
-
(D1)
has no accumulation points in ℂ;
-
(D2)
for each , no subsequence of is normal at ;
-
(D3)
in .
Obviously, and all zeros of have multiplicity at least in .
Clearly, , so is meromorphic in . By Lemma 3.1 and (C4), either or in .
Subcase 1.1.1.1. is an empty set.
By (C2), we have . If in ℂ, then by Lemma 2.8 and Lemma 2.9, is a constant function which contradicts that . If in ℂ, then , which contradicts .
Subcase 1.1.1.2. is not an empty set.
Let . Since , by Lemma 2.5, we have in Clearly, and are meromorphic functions in , so we have in , which contradicts .
Subcase 1.1.2. .
By Lemma 2.5, we have in . If , then is a multi-valued function in . A contradiction. Thus we have .
We claim that . Otherwise, there exists such that and . By Lemma 2.5,
By (5.2) and (5.3), we obtain , which contradicts . Thus .
By Lemma 2.5,
in , where is a polynomial of degree . By (5.4),
in Δ. By Hurwitz’ theorem, there exist () such that and . Since , we have , and hence for .
Set and , where is one of of largest modulus. Clearly, . Now, we have:
-
(E1)
for each , in for sufficiently large n;
-
(E2)
;
-
(E3)
and in ℂ;
-
(E4)
has only poles on , where .
In fact, (E1) holds by (5.5). By Lemma 3.3, (E1), and (E3), we find that is normal in ℂ. We assume that in ℂ. Obviously, by (E2).
Subcase 1.1.2.1. is a meromorphic function in ℂ.
By Lemma 3.1, we have either or in ℂ. Since , in ℂ. By Lemma 2.8 and Lemma 2.9, is a constant function. This contradicts .
Subcase 1.1.2.2. in ℂ.
Set . By the maximum principle applied to , we get that
Set . By (5.5), has no poles in for sufficiently large n. By the maximum principle,
Hence,
On the other hand,
in ℂ, and hence , which contradicts (5.7).
Subcase 1.2. There exists a subsequence of (still denoted by ) such that for each n.
Do as in Subcase 1.1.1, we may assume that (≠0) is the pole of of smallest modulus and . Set . We have:
-
(F1)
is holomorphic function in Δ;
-
(F2)
;
-
(F3)
all zeros of have multiplicity at least ;
-
(F4)
and in ℂ.
By Lemma 3.8 and Lemma 3.4, is normal in Δ and quasinormal in ℂ. Thus, there exist a subsequence of (still denoted by ) and such that:
-
(G1)
has no accumulation points in ℂ;
-
(G2)
for each , no subsequence of is normal at ;
-
(G3)
in .
Obviously, and all zeros of have multiplicity at least in .
Obviously, , so is a meromorphic function in . By Lemma 3.1 and (F4), either or in .
Subcase 1.2.1. is an empty set.
By (F2), . If , then by Theorem 2.8 and Lemma 2.9, is a constant function, which contradicts that . If , then we have , which contradicts .
Subcase 1.2.2. is not an empty set.
Let . Since , by Lemma 2.5, we have , which contradicts .
Case 2. .
In this case, we will show that (∗f) and (∗g) hold. Clearly, we have in , in , and .
We claim that for each , there exists at least one zero of in for sufficiently large n. Otherwise, there exist (>0) and a subsequence of (still denoted by ) such that in . Since and all the zeros of have multiplicity at least , we have , and hence in . By Lemma 3.3, is normal at 0, which contradicts (∗c).
Taking a subsequence and renumbering, we may assume that (≠0) is the zero of of smallest modulus and . Set . We have:
-
(a1)
in Δ;
-
(a2)
all zeros of have multiplicity at least and ;
-
(a3)
and in ℂ.
By Lemma 3.3 and Lemma 3.4, is normal in Δ and quasinormal in ℂ. Thus, there exist a subsequence of (still denoted by ) and such that:
-
(b1)
has no accumulation points in ℂ;
-
(b2)
for each , no subsequence of is normal at ;
-
(b3)
in .
Obviously, and all zeros of have multiplicity at least in .
Subcase 2.1. .
By (a2), , and hence is a meromorphic function in .
We claim that . Otherwise, let . Since , by Lemma 2.5, , which contradicts that .
By Lemma 3.1 and (a3), either or in ℂ. Since , we have in ℂ. By Lemma 2.8 and Lemma 3.5, we have
where is an integer, , and (). Thus, we have
By Hurwitz’s theorem, there exist sequences and (counting multiplicities of zeros and poles, respectively) such that for sufficiently large n, and , where and . set . Thus, and , where . Set
Subcase 2.1.1. For each , has at least zeros (counting multiplicities) in for sufficiently large n.
Taking a subsequence and renumbering, we may assume that (≠0) is the zero of of smallest modulus in and . Clearly, and for . By Hurwitz’s theorem and (5.8), as . Set . We have, for sufficiently large n:
-
(c1)
has only zeros in Δ. Obviously, ;
-
(c2)
all zeros of have multiplicity at least and ;
-
(c3)
and in ℂ.
By Lemma 3.3 and Lemma 3.4, is normal in and quasinormal in ℂ. Thus, there exist a subsequence of (still denoted by ) and such that:
-
(d1)
has no accumulation points in ℂ;
-
(d2)
for each , no subsequence of is normal at ;
-
(d3)
in .
Obviously, and all zeros of have multiplicity at least in .
Set
By (5.8), in ℂ. Hence
Subcase 2.1.1.1. in .
Obviously, has no zeros in Δ for sufficiently large n. Applying the maximum principle to , we see that in Δ, which contradicts (5.9).
Subcase 2.1.1.2. is a meromorphic function in .
We claim that
By Lemma 3.2, in , and then in , where is a polynomial of degree . Since has no zeros in Δ for sufficiently large n, applying the maximum principle to , in Δ. By (5.9),
Since is a polynomial of degree , we have by (5.11). Thus our claim is proved.
Suppose that . By (c2), we have , which contradicts (5.10). Suppose that . By Lemma 2.5, . However, we have by (5.10). This is a contradiction.
Subcase 2.1.2. There exists such that has exactly zeros (counting multiplicities) in for sufficiently large n.
Taking a subsequence and renumbering, we may assume that has exactly zeros (counting multiplicities) in for all n. Now, (∗f) holds with and . Next, we will show that (∗g) also holds.
Set
Clearly, has no zeros in . By (5.8), in ℂ, and hence
Subcase 2.1.2.1 in .
By the maximum principle applied to , we have in , which contradicts (5.12).
Subcase 2.1.2.2. is a meromorphic function in .
By Lemma 3.2, in . By the maximum principle applied to , in . Hence by (5.12),
Equation (5.13) implies that is a zero of order of . Thus we have
Subcase 2.2. .
By Lemma 2.5,
We claim that . Otherwise, there exists and . By Lemma 2.5, in ,
By (5.14) and (5.16), we obtain . Furthermore, by Lemma 2.5, in ,
By (5.15) and (5.17), we get . A contradiction. Thus .
By Lemma 2.5, can extend to an analytic function in ℂ and thus is a polynomial of degree . Since 1 is a zero of order k of F, F must have at least one zero which is distinct from 1. Let (≠1) is a zero of . Since all zeros of have multiplicity at least in , we have and . Then is a multiple zero of . However, by (5.14), only has zeros of order 1. This is a contradiction.
Case 3. .
Obviously, (∗g) holds by Lemma 2.5. Next, we will show that (∗f) also holds.
Since is not normal at 0, it follows from Lemma 2.2 that we can extract a subsequence (still denoted by ), points , and positive numbers such that
where g is a nonconstant meromorphic function of finite order in ℂ, all of whose zeros have multiplicity at least . Since and on the complement of the poles of , either or in ℂ by Hurwitz’s theorem. The latter case is not possible, as this would contradict the fact that all zeros of g have multiplicity at least . Thus in ℂ. By Lemma 2.11,
for distinct complex numbers a and b. It now follows from the argument principle that there exists a sequence such that, for sufficiently large n, . Thus, writing , we have . By (5.18) and (5.19), the multiplicity of as a zero of is exactly for sufficiently large n. By Lemma 2.5, there exists such that has a single (multiple) zero in for sufficiently large n. Thus for sufficiently large n, has a single zero of order exactly in . Now, (∗f) holds with and . □
6 Proof of Proposition 1.2
Proof By Lemma 3.4, is quasinormal in D. Hence for each subsequence of , there exists a subsequence of (still denoted by ) and a corresponding point set E which has no accumulation points in D such that:
-
(a)
in , where is meromorphic or identically infinite there;
-
(b)
for each , no subsequence of is normal at .
By Lemma 3.9, we have:
-
(c)
for each , ;
-
(d)
for each , there exist and such that for sufficiently large k, , where and only depend on ;
-
(e)
for each , in .
Clearly, we see that (1), (2), (3), and (4) hold.
Suppose that and are distinct points in E. Since for each , , there exist such that and . However, and by (e). Thus . A contradiction. This completes the proof of Proposition 1.2. □
7 Proof of Theorem 1.3
Proof We argue by contradiction. Suppose that the equation has at most finitely many solutions. Let as , where and .
Clearly, and as . By Lemma 2.1, as , where is a constant. By standard results in Nevanlinna theory, and as . Thus, as . Since as , we have .
Set . By Lemma 3.6, there exist points and positive numbers such that
and
Let , be the two fundamental periods of and P () be a fundamental parallelogram of . There exist integers and such that , where . There exists a subsequence of (still denoted by ) such that as . Set
Clearly, we have , , and . By (7.1) and (7.2), we have
There exists such that and for each n. Set . Obviously, we have . By assumption, for sufficiently large n,
For each ,
So we have
Set
Obviously, in D, and for sufficiently large n, and have the same zeros and poles with the same multiplicity in D.
Clearly, is a family of meromorphic functions in D such that for sufficiently large n:
-
(a1)
all zeros of have multiplicity at least in D;
-
(a2)
in D, where in D;
-
(a3)
in D.
It follows from Lemma 3.4 that is quasinormal in D. Hence there exists such that and is normal in . Then there exists a subsequence of (still denoted by ) such that:
-
(b1)
and have the same zeros and poles with the same multiplicity in ;
-
(b2)
for all , in ;
-
(b3)
no subsequence of is normal at ;
-
(b4)
all zeros of have multiplicity at least in , and in .
By (7.6), (b3) holds. By Lemma 3.9, we have:
-
(c1)
;
-
(c2)
there exist and such that for sufficiently large n;
-
(c3)
in .
By Lemma 2.3, (c2) and (c3), there exists such that for sufficiently large n,
Next, we will derive a contradiction with (7.5).
By (7.3) and (7.4), . Then
so
Using the simple inequality
for , we have
The second term on the right of (7.8) is
Putting (7.8), (7.9), and (7.10) together, we have for and sufficiently large n,
It follows from (7.7) and (7.11) that
which contradicts (7.5). This completes the proof of Theorem 1.3. □
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 11001081) and the Scientific Research Foundation of CUIT (No. KYTZ201403).
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Yang, P. A quasinormal criterion of meromorphic functions and its application. J Inequal Appl 2014, 389 (2014). https://doi.org/10.1186/1029-242X-2014-389
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DOI: https://doi.org/10.1186/1029-242X-2014-389