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Zeros of some difference polynomials
Advances in Difference Equations volume 2013, Article number: 194 (2013)
Abstract
In this paper, we study zeros of some difference polynomials in and their shifts, where is a finite order meromorphic function having deficient value ∞. These results improve previous findings.
1 Introduction and results
In this paper, we use the basic notions of Nevanlinna’s theory (see [1]). In addition, we use the notations to denote the order of growth of the meromorphic function , and to denote the exponent of convergence of zeros of .
Hayman [2] studied the Picard values of a meromorphic function and their derivatives, and he obtained the following result.
Theorem A If is a transcendental entire function, is an integer, and a (≠0) is a constant, then assumes all finite values infinitely often.
Yang [3, 4] extended Theorem A to some differential polynomial in , where is a transcendental meromorphic function satisfying .
Recently, many papers have focused on complex differences. They have obtained many new results on difference utilizing the value distribution theory of meromorphic functions. Some findings may be viewed as the corresponding difference analogues. First, recall some definitions. We call
is a monomial in and its shifts , where , are distinct nonzero complex constants, and is its degree. Further,
is called a difference polynomial in and its shifts, and is its degree, where I is a finite set of the index , and are meromorphic coefficients being small with respect to f in the sense that , .
Zheng and Chen [5] obtained the following theorem, which may be considered as a difference counterpart of Theorem A.
Theorem B Let be a transcendental entire function of finite order, and let a, c be nonzero constants. Then, for any integer , assumes all finite values infinitely often.
Chen [6] obtained the following Theorems C-E, which may be considered as another difference counterpart of Theorem A.
Theorem C Let be a transcendental entire function of finite order, and let be constants with c such that . Set , where and is an integer. Then assumes all finite values infinitely often, and for every one has .
Theorem D Let be a transcendental entire function of finite order with a Borel exceptional value 0, and let be constants, with c such that . Then assumes all finite values infinitely often, and for every one has .
Zheng and Chen [7] extended the above Theorems B-D to a difference polynomial and proved the following theorem.
Theorem E Let be a transcendental meromorphic function of finite order satisfying
Suppose that is a difference polynomial of the form (1.1) and contains exactly one term of maximal total degree in and its shifts. Then, for any given , the difference polynomial
satisfies . Therefore, takes every nonzero finite value infinitely often.
We observe that condition (1.2) can be weakened. It is well known that deficient value plays an important role in the theory of value distribution of meromorphic functions. In this paper, we consider the meromorphic function having deficient value ∞ and obtain the following improvements of Theorem E, which may be stated as follows.
Theorem 1.1 Let be a transcendental meromorphic function of finite order, and let (≢0) be a difference polynomial of the form (1.1) with k different shifts. Assume that . Then the difference polynomial
has infinitely many zeros and satisfies .
If we add the condition , then the condition in Theorem 1.1 can be weakened as , and we obtain the following theorem.
Theorem 1.2 Let be a transcendental meromorphic function of finite order, and let (≢0) be a difference polynomial of the form (1.1) with k different shifts. Assume that and , then the difference polynomial
has infinitely many zeros and satisfies .
Example 1.1 For , , we have . Here , which shows that the condition in Theorem 1.2 cannot be replaced with , where c is a nonzero constant.
If we add the additional condition , then we obtain the following.
Theorem 1.3 Let be a transcendental meromorphic function of finite order satisfying
Then the difference polynomial defined by (1.3) has infinitely many zeros and satisfies .
Theorem 1.4 Let be a transcendental meromorphic function of finite order with satisfying
Then the difference polynomial defined by (1.4) has infinitely many zeros and satisfies .
Remark 1.1 For any given , applying Theorems 1.1-1.4 to the difference polynomial , then we have .
Remark 1.2 If , where is small with respect to f, applying Theorems 1.1-1.4 to the difference polynomial , then we also have .
Applying Theorems 1.1-1.4 to the difference polynomial , we get the following Corollaries 1.5-1.8, which extend Theorems C, D.
Corollary 1.5 Let be a transcendental meromorphic function of finite order with , and let be constants with c such that does not reduce to any constant and is an integer. Then, for every , one has .
Corollary 1.6 Let be a transcendental meromorphic function of finite order satisfying
and let be constants, with c such that does not reduce to any constant and is an integer. Then, for every , one has .
Corollary 1.7 Let be a transcendental meromorphic function of finite order satisfying
and let be constants, with c such that does not reduce to any constant. Then, for every , one has .
Corollary 1.8 Let be a transcendental meromorphic function of finite order satisfying and
and let be constants with c such that does not reduce to any constant. Then, for every , one has .
2 Lemmas for the proof of the theorem
Lemma 2.1 [8]
Given two distinct complex constants , , let f be a meromorphic function of finite order. Then we have
Lemma 2.2 [9]
Let be a nonconstant meromorphic function of finite order, then for any given constant c (≠0), we have
possibly outside of an exceptional set of finite logarithmic measure.
Lemma 2.3 [4]
Let be a transcendental meromorphic function, and let be an algebraic polynomial in f of the form
where , () are small with respect to f. Then
Using the same method as in the proof of Lemma 2.3, we show the following.
Lemma 2.4 Let be a transcendental meromorphic function with , and let be an algebraic polynomial in f of the form (2.1), where , satisfy , , then
Proof Since satisfy , , then
Thus,
□
Lemma 2.5 [10]
Let f be a transcendental meromorphic solution of finite order of a difference equation of the form
where , and are difference polynomials such that the total degree in and its shifts, and . Moreover, we assume that all coefficients in (2.2) are small in the sense that and that contains just one term of maximal total degree in and its shifts. Then, for each , we have
possibly outside of an exceptional set of finite logarithmic measure.
Yang and Laine in [11] further pointed out the following.
Remark 2.1 If and are differential-difference polynomials in and its shifts with coefficients satisfy , then Lemma 2.5 still holds.
3 Proof of Theorems 1.1-1.4
Proof of Theorem 1.1 We assert that does not reduce to any constant. In fact if , where is some constant, then
Applying Lemma 2.5 to (3.1), since , we have
Since , then for any given () and sufficiently large r, we have
By (3.2) and (3.3), we get
which is impossible. So, does not reduce to any constant.
Next we rearrange the expression difference polynomial by collecting together all terms having the same total degree and then writing in the form
where the coefficients , are the sum of finitely many terms of the type
By Lemma 2.1 and our assumption concerning the coefficients , we obtain
By (3.4), (3.5) and Lemma 2.4, we have
By Lemma 2.2, we have
and
Thus,
So, .
Differentiating both sides of (1.3), we obtain that
Multiplying both sides of (1.3) by and subtracting from (3.8), we get
or
Now assert . Otherwise , i.e., for a suitable constant leading to
We can see that , for . Since by assumption and Lemma 2.5, (3.11) implies , which shows that , again by (3.3). It is a contradiction.
Applying Lemma 2.5 and Remark 2.1 to (3.9) and (3.10) respectively, again by assumption , we obtain
and
By (3.6), we get that
and
By (3.12)-(3.15), we obtain
and
Therefore,
or
For any given ε (), we have
or
By (3.7) and (3.19), we have
which leads to
Therefore,
□
Proof of Theorem 1.3 Using the same method as in the proof of Theorem 1.1, we see that (3.1)-(3.18) hold. By assumption and (3.16)-(3.18), we have
For any given ε, , where , we have
By (3.7), we have
which leads to
Therefore,
□
Proof of Theorem 1.2 The proof is similar to the proof of Theorem 1.1.
We also obtain (3.1)-(3.9), (3.12), (3.14) and (3.16), by assumption that .
By logarithmic derivative lemma, we get
Since , by (3.6), we get
By (3.20) and (3.21), we have
Then,
or
For any given ε, , where , by (3.7), we obtain
which leads to
Therefore,
□
Proof of Theorem 1.4 Using the same method as in the proof of Theorem 1.2, we obtain (3.23). By (1.5) and (3.23), we have
For any given ε, , where , we have
Then by (3.7), we have
which leads to
Therefore,
□
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Acknowledgements
The authors would like to thank the editor and the referees for their constructive comments to improve the readability of our paper. The project was supported by the National Natural Science Foundation of China (11171119, 11126145).
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Lan, S., Chen, Z. Zeros of some difference polynomials. Adv Differ Equ 2013, 194 (2013). https://doi.org/10.1186/1687-1847-2013-194
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DOI: https://doi.org/10.1186/1687-1847-2013-194