Abstract
We investigate the value distribution of difference operator for meromorphic functions. In addition, we study the sharing value problems related to a meromorphic function f (z) and its shift f (z + c).
Similar content being viewed by others
1 Introduction and main results
A meromorphic function means meromorphic in the whole complex plane. We assume that the reader is familiar with standard symbols and fundamental results of Nevanlinna Theory [1]. As usual, the abbreviation CM stands for "counting multiplicities", while IM means "ignoring multiplicities", and we denote the order of meromorphic function f by σ (f). For a non-constant meromorphic function f and a set S of complex numbers, we define the set E(S, f) = ∪a∈S{z|f(z) - a = 0}, where a zero of f - a with multiplicity m counts m times in E(S, f).
We define difference operator as Δ c f = f (z + c) - f (z), where c is a non-zero constant. In particular, we denote by S (f) the family of all meromorphic functions a (z) that satisfy T(r, a) = S(r, f) = o(T(r, f)), where r → ∞ outside a possible exceptional set of finite logarithmic measure. For convenience, we set Ŝ(f) := S(f) ∪ {∞}.
The difference Nevanlinna theory and its applications to the uniqueness theory have become a subject of great interest [2–4], recently. With these fundamental results, Heittokangas et al. considered a meromorphic function f (z) sharing values with its shift f(z + c), we recall a key result from [5].
Theorem A [[5], Theorem 2]. Let f be a non-constant meromorphic function of finite order, let c ∈ ℂ, and let a, b, c ∈ Ŝ(f) be three distinct periodic functions with period c. If f (z) and f (z + c) share a, b CM and c IM, then f (z) = f (z + c) for all z ∈ ℂ.
Recently, Yang and Liu and one of the present authors [6] considered the case F = fn, where f is a meromorphic function, assuming value sharing with F and F (z + c):
Theorem B [[6], Theorem 1.4]. Let f be a non-constant meromorphic function of finite order, n ≥ 7 be an integer, let c ∈ ℂ, and let F = fn. If F (z) and F (z + c) share a ∈ S(f)\{0} and ∞ CM, then f (z) = ωf (z + c), for a constant ω that satisfies ωn= 1.
Next, we consider the problem that related to the Theorem B, and have the following result, where a is a periodic function with period c. However, our proof is different to the one in [6].
Theorem 1.1. Let f be a non-constant meromorphic function of finite order, let c ∈ ℂ, and let a ∈ S(f) \ {0} be a periodic function with period c. If f (z)nand f(z + c)nshare a and ∞ CM, and n ≥ 4 is an integer, then f (z) = ωf (z + c), for a constant ω that satisfies ωn= 1.
Remarks.
-
(1)
Theorem 1.1 is not true, if a = 0. This can be seen by considering . Then f (z)nand f (z + c)nshare 0 and ∞ CM, however, f (z) ≠ ωf (z + c), where n is a positive integer.
-
(2)
Theorem 1.1 does not remain valid when n = 1. For example, f (z) = ez+ 1 and f (z + c) = ez+c+ 1, where c ≠ 2πi. Clearly, f(z) and f (z + c) share 1 and ∞ CM, however, f (z) ≠ ωf (z + c) for ωn= 1. Unfortunately, we have not succeeded in reducing the condition n ≥ 4 to n ≥ 2 in Theorem 1.1, and we also cannot give a counterexample when n = 2, 3 at present.
-
(3)
We give an example to show that the restriction of finite order in Theorem 1.1 cannot be deleted. This can be seen by taking . Then f (z)nand f (z + c)nshare 1 and ∞ CM, however, f(z) ≠ ωf (z + c), where n is a positive integer.
In 1976, Gross asked the following question [[7], Question 6]:
Question. Can one find (even one set) finite sets S j (j = 1, 2) such that any two entire functions f and g satisfying E(S j , f) = E(S j , g) (j = 1, 2) must be identical?
Since then, many results have been obtained for this and related topics (see [8–11]). We recall the following result given by Yi [9].
Theorem C [[9], Theorem 1]. Let S1 = {ω | ωn+ aωn- 1+ b = 0}, where n ≥ 7 is an integer, a and b are two non-zero constants such that the algebraic equation ωn+ aωn- 1+ b = 0 has no multiple roots. If f and g are two entire functions satisfying E(S1, f) = E(S1, g), then f = g.
Afterwards, Fang and Lahiri [12] got the result for meromorphic functions.
Theorem D [[12], Theorem 1]. Let S1 be defined as Theorem C and S2 = {∞}. Assume that f and g are two meromorphic functions satisfying E(S j , f) = E (S j , g) for j = 1,2. If f has no simple poles and n ≥ 7, then f = g.
Next, we give a difference analog of Theorem D that replacing g with f (z + c), and obtain the following result.
Theorem 1.2. Let S1 be defined as Theorem C and S2 = {∞}. Assume that f is a meromorphic function of finite order satisfying E(S j , f) = E(S j , f (z + c)) for j = 1,2. If n ≥ 6 and , then f (z) = f (z + c) for all z ∈ ℂ.
We investigate the value distribution of difference polynomials of meromorphic (entire) functions. Let f be a transcendental meromorphic function, and let n be a positive integer. Concerning to the value distribution of fnf", Hayman [[13], Corollary to Theorem 9] proved that fnf' takes every non-zero complex value infinitely often if n ≥ 3. Mues [[14], Satz 3] proved that f2f' - 1 has infinitely many zeros. Later on, Bergweiler and Eremenko [[15], Theorem 2] showed that ff' - 1 has infinitely many zeros also. As an analog result in difference, Laine and Yang [16] investigated the value distribution of difference products of entire functions, and obtained the following:
Theorem E [[16], Theorem 2]. Let f be a transcendental entire function of finite order, and let c be a non-zero complex constant. Then for n ≥ 2, f (z)nf (z + c) assumes every non-zero value a ∈ ℂ infinitely often.
In a recent article, one of the present authors considered the value distribution of f (z)n(f(z) - 1) f (z + c), the result may be stated as follows:
Theorem F [[17], Theorem 1]. Let f be a transcendental meromorphic function of finite order σ(f), let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. If the exponent of convergence of the poles of f satisfies and n ≥ 2, then f (z)n(f - 1) f (z + c) - a has infinitely many zeros.
In this article, we replace f (z + c) with Δ c f, and consider the value distribution of f (z)n(f(z) - 1)Δ c f. We get the following results:
Theorem 1.3. Let f be a transcendental meromorphic function of finite order σ(f) and Δ c f ≠ 0, let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. If the exponent of convergence of the poles off satisfies and n ≥ 3, then f (z)n(f - 1)Δ c f - a has infinitely many zeros.
Corollary 1.4. Let f be a transcendental entire function of finite order and Δ c f ≠ 0, let a ≠ 0 be a small function with respect to f, and let c be a non-zero complex constant. Then for n ≥ 3, f (z)n(f - 1)Δ c f - a has infinitely many zeros.
In particular, if a is a non-zero polynomial in Corollary 1.4, then Corollary 1.4 can be improved.
Theorem 1.5. Let f be a transcendental entire function of finite order and Δ c f ≠ 0, let a be a non-zero polynomial, and let c be a non-zero complex constant. Then for n ≥ 2, f (z)n(f - 1)Δ c f - a has infinitely many zeros.
2 Preliminary lemmas
Lemma 2.1. [[4], Theorem 2.1] Let f be a meromorphic function of finite order, and let c ∈ ℂ and δ ∈ (0, 1) . Then
Chiang and Feng have obtained similar estimates for the logarithmic difference [[3], Corollary 2.5], and this study is independent from [4].
Lemma 2.2. [[4], Lemma 2.3] Let f be a meromorphic function of finite order and c ∈ ℂ. Then for any small function a ∈ S (f) with period c,
Lemma 2.3. [[3], Theorem 2.1] Let f be a meromorphic function of finite order σ (f), and let c be a non-zero constant. Then, for each ε > 0, we have
Lemma 2.4. [[18], Theorem 2.4.2] Let f be a transcendental meromorphic solution of
where A(z, f), B(z, f) are differential polynomials in f and its derivatives with small meromorphic coefficients a λ , in the sense of m(r, a λ ) = S(r, f) for all λ ∈ I. If the deg(B(z, f)) ≤ n, then m(r, A(z, f)) = S(r, f).
Lemma 2.5. Let f be a finite order entire function and Δ c f ≠ 0, and let c be a non-zero constant. Then
Proof. Since f is an entire function with finite order, we deduce from Lemma 2.2 and the Lemma of logarithmic derivative that
Hence, we get
3 Proof of Theorem 1.1
Since f(z)nand f(z + c)nshare a and ∞ CM, we obtain that
where Q(z) is a polynomial. From Lemma 2.1, we know that T (r, eQ(z)) = m (r, eQ(z)) = S (r, f). Rewrite (2) as
Set
If eQ(z)≢ 1, then we apply the Valiron-Mohon'ko theorem and the second main theorem to G (z), and get
Combining (4) with Lemma 2.3, we get
which contradicts that n ≥ 4. Therefore, eQ(z)≡ 1, that is, f (z)n= f (z + c)n, so we have f (z) = ω f (z + c), for a constant ω with ωn= 1.
4 Proof of Theorem 1.2
From the assumption of Theorem 1.2, we get that
where Q(z) is a polynomial. Applying Lemma 2.1, we obtain that T (r, eQ(z)) = m (r, eQ(z)) = S (r, f). Rewrite (5) as
If eQ(z)≢ 1, applying the second main theorem for three small functions, we get
Combining (4.3) with Lemma 2.3, we get
which contradicts n ≥ 6. Hence, eQ(z)≡ 1, we conclude by (5) that
Set . If G (z) is non-constant, then we have from (8)
Making use of the standard Valiron-Mohon'ko lemma, we get from (9) that
Noting that n ≥ 6, we deduce that 1 is not a Picard value of Gn. Suppose that a j ∈ {ℂ \ 1} (j = 1, 2,..., n - 1) are the distinct roots of equation hn- 1 = 0. Applying the second main theorem to G, we conclude by (9) that
From (10) and (11), we get , which contradicts the assumption.
So G(z) is a constant, and we get f (z) = tf (z + c), where t is a non-zero constant. From (8), we know t = 1, therefore, f = g.
5 Proof of Theorem 1.3
The main idea of this proof is from [[17], Theorem 1], while the details are somewhat different. For the convenience of the reader, we give a complete proof.
Set F(z) = fn(z) (f(z) - 1)Δ c f. Since f is a transcendental meromorphic function with finite order σ(f), we conclude by Lemma 2.3 that
Thus, we get S(r, F) = o(T(r, f)) = S(r, f). Moreover, we get
On the other hand, we deduce by Lemma 2.2 that
Hence
The second main theorem yields
From (12) and above inequality, we get that
Combining (13) and (14), we have
which is a contradiction to the fact that f is of order σ (f) if F - a has finitely many zeros. The conclusion follows.
6 Proof of Theorem 1.5
Suppose that fn(f - 1)Δ c f - a admits finitely many zeros only. Then, there are two non-zero polynomials P(z), Q(z) such that
Differentiating (15) and eliminating eQ(z), we obtain
where
and P*(z) = P'(z) + P(z)Q'(z).
First, we conclude that a'P (z) - aP*(z) ≢ 0. Otherwise, if a'P(z) - aP*(z) = 0, by integrating, then we have
where A is a non-zero constant. Hence, we get eQ(z)is a constant and
where B is a non-zero constant. Then, from Lemma 2.3 and (17), we obtain that
which is a contradiction when n ≥ 2.
If F(z, f) vanish identically, then
Rewrite (18), we get
hence
Then, combining Lemmas 2.2, 2.4 and Equation (19), we conclude that
which contradicts (1).
It remains to consider the case that F (z, f) ≢ 0. We rewrite (16) in the form that
and
By Lemmas 2.2 and 2.4, we know that
and
As f (z) is entire, we get that the poles of may be located only at the zeros of f (z). If has infinitely many poles, then from that a zero of f (z) with multiplicity t should be a pole of t + 1 of . Since n ≥ 2, we know that the left side of (20) must have infinitely many zeros, which is a contradiction that a'P(z) - aP*(z) is a non-zero polynomial. We get
Hence
and
as well. Combining these two estimates, we obtain
contradiction. This completes the proof of Theorem 1.5.
References
Yang CC, Yi HX: Uniqueness Theory of Meromorphic Functions. Kluwer Academic Publishers, Dordrecht; 2003.
Bergweiler W, Langley JK: Zeros of difference of meromorphic functions. Math Proc Cambridge Philos Soc 2007, 142: 133–147. 10.1017/S0305004106009777
Chiang YM, Feng SJ: On the Nevanlinna characteristic of f ( z + η ) and difference equations in the complex plane. Ramanujan J 2008, 16: 105–129. 10.1007/s11139-007-9101-1
Halburd RG, Korhonen RJ: Nevanlinna theory for the difference operator. Ann Acad Sci Fenn 2006, 31: 463–478.
Heittokangas J, Korhonen R, Laine I, Rieppo J, Zhang JL: Value sharing results for shifts of meromorphic function, and sufficient conditions for periodicity. J Math Anal Appl 2009, 355: 352–363. 10.1016/j.jmaa.2009.01.053
Qi XG, Yang LZ, Liu K: Uniqueness and periodicity of meromorphic functions concerning difference operator. Comput Math Appl 2010, 60: 1739–1746. 10.1016/j.camwa.2010.07.004
Gross F: Factorization of meromorphic functions and some open problems, Complex analysis (Proc. Conf., Univ. Kentucky, Lexington, Ky., 1976). In Lecture Notes in Math. Volume 599. Springer, Berlin; 1977:51–67. 10.1007/BFb0096825
Frank G, Reinders M: A unique range set for meromorphic functions with 11 elements. Complex var Theory Appl 1998, 37: 185–193. 10.1080/17476939808815132
Yi HX: A question of Gross and the uniqueness of entire functions. Nagoya Math J 1995, 138: 169–177.
Yi HX: Unicity theorems for meromorphic or entire functions II. Bull Austral Math Soc 1995, 52: 215–224. 10.1017/S0004972700014635
Yi HX: Unicity theorems for meromorphic or entire functions III. Bull Austral Math Soc 1996, 53: 71–82. 10.1017/S0004972700016737
Fang ML, Lahiri I: Unique range set for certain meromorphic functions. Indian J Math 2003, 45: 141–150.
Hayman WK: Picard values of meromorphic functions and their derivatives. Ann Math 1959, 70(2):9–42.
Mues E: Über ein Problem von Hayman. Math Z 1979, 164: 239–259. 10.1007/BF01182271
Bergweiler W, Eremenko A: On the singularities of the inverse to a meromorphic function of finite order. Rev Mat Iberoamericana 1995, 11: 355–373.
Laine I, Yang CC: Value Distribution of Difference Polynomials. Proc Japan Acad Ser A Math Sci 2007, 83: 148–151. 10.3792/pjaa.83.148
Qi XG: Value distribution and uniqueness of difference polynomials and entire solutions of difference equations. Ann Polon Math 2011, 102(2):129–142. 10.4064/ap102-2-3
Laine I: Nevanlinna Theory and Complex Differential Equations. In Walter de Gruyter. Berlin-New York; 1993.
Acknowledgements
The authors thank the referee for his/her valuable suggestions to improve the present article. This study was supported by the NSF of Shandong Province, China (No. ZR2010AM030).
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
XQ completed the main part of this article, JD and LY corrected the main theorems. All authors read and approved the final manuscript.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Qi, X., Dou, J. & Yang, L. Uniqueness and value distribution for difference operators of meromorphic function. Adv Differ Equ 2012, 32 (2012). https://doi.org/10.1186/1687-1847-2012-32
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1687-1847-2012-32