# The existence of solutions of *q*-difference-differential equations

- Xin-Li Wang
^{1}, - Hua Wang
^{2}and - Hong-Yan Xu
^{2}Email author

**Received: **12 November 2015

**Accepted: **19 April 2016

**Published: **4 May 2016

## Abstract

By using the Nevanlinna theory of value distribution, we investigate the existence of solutions of some types of non-linear *q*-difference differential equations. In particular, we generalize the Rellich–Wittich-type theorem and Malmquist-type theorem about differential equations to the case of *q*-difference differential equations (system).

### Keywords

Transcendental*q*-Difference differential equation Solution Zero order

### Mathematics Subject Classification

39A 50 30D 35## Background

*m*(

*r*,

*f*), \(N(r,f), T(r,f), \ldots\), (see Hayman 1964; Yang 1993; Yi and Yang 1995). For a meromorphic function

*f*,

*S*(

*r*,

*f*) denotes any quantity satisfying \(S(r,f)=o(T(r,f))\) for all

*r*outside a possible exceptional set of finite logarithmic measure, \(\mathbb {S}(f)\) denotes the family of all meromorphic function

*a*(

*z*) such that \(T(r,a)=S(r,f)=o(T(r,f))\), where \(r\rightarrow \infty\) outside of a possible exceptional set of finite logarithmic measure. In addition, we denote by \(S_1(r,f)\) any quantity satisfying \(S_1(r,f)=o(T(r,f))\) for all

*r*on a set

*F*of logarithmic density 1, the logarithmic density of a set

*F*is defined by

*F*of logarithmic density can be not necessarily the same at each occurrence.

Complex differential equations have attracted many mathematicians, and there are many results about the existence or growth of solutions of differential equations (see He 1981; Laine 1993, 1971; Liao 2015; Tu et al. 2013). In recent, with the development of Nevanlinna theory in complex difference equations (see Barnett et al. 2007; Chiang and Feng 2008; Gundersen et al. 2002; Halburd and Korhonen 2006a, b), there has been an increasing interest in studying difference equations, difference product and *q*-difference in the complex plane \(\mathbb {C}\), a number of papers (including Chen 2010; Gan 2015; Halburd and Korhonen 2007; Heittokangas et al. 2001; Laine and Yang 2007; Qi and Yang 2015; Zheng and Chen 2010; Zhang and Korhonen 2010) have focused on the existence and growth of solutions of difference equation.

The following two results had been proved by F. Rellich and H. Wittich, respectively.

###
**Theorem 1**

Wittich (1955) studied the more general differential equation than Eq. (1) and obtained the following result.

###
**Theorem 2**

*Let*

*be differential polynomial, with coefficients*\(a_{(i)}(z)\)

*are polynomial of*

*z*.

*If the right-hand side of the differential equation*

*f(w)*

*is the transcendental meromorphic function of*

*w*

*, then the Eq. (*2

*) has no non-constant entire solution.*

In the 1980s, Yanagihara and Shimomura extended the above type theorem to the case of difference equations (see Yanagihara 1980, 1983; Shimomura 1981), and obtained the following two results

###
**Theorem 3**

*For any non-constant polynomial*

*P*(

*w*),

*the difference equation*

*has a non-trivial entire solution.*

###
**Theorem 4**

*For any non-constant rational function*

*R(w),*

*the difference equation*

*has a non-trivial meromorphic solution in the complex plane.*

## Conclusions and our main results

In the present paper, we mainly study the above Rellich–Wittich-type theorem of *q*-difference differential equation (system).

###
**Definition 5**

We call the equation a *q*-difference differential equation (system) if a equation (system) contains the *q*-difference term *f*(*qz*) and differential term \(f'(z)\) of one function *f*(*z*) at the same time.

*q*-difference differential equation of the form

*z*and \(q\in \mathbb {C}{\setminus }\{0\}\), \(P_m[f]\) is a polynomial of

*f*of degree

*m*,

*z*, and obtain the following results.

###
**Theorem 6**

*For system* (3), *if*
\(s\ge 1, t\ge 1\) and *f*
*is a transcendental meromorphic function, then the system* (3) *has no non-constant transcendental entire solutions*
\((w_1,w_2)\)
*with zero order.*

###
*Remark 7*

*q*-difference differential equation

*f*.

If \(s=t\) and \(w_1=w_2\), we can get the following theorem easily

###
**Theorem 8**

*Let*

*if*\(s\ge 1\)

*and*

*f*

*is a transcendental meromorphic function, then*

*the system*(4)

*has no non-constant transcendental entire*

*solution with zero order.*

From Remark 7, we have

###
*Remark 9*

*f*be a transcendental meromorphic function, then the equation

*f*.

As we know, it is very interest problem about the Malmquist theorem of differential equations, Laine (1993) gave the following results

###
**Theorem 9**

###
**Theorem 10**

*Let*

*where*

*R*(

*z*,

*w*)

*is defined as in Theorem*9.

*If Eq.*(6)

*has transcendental meromorphic solution, then there will be*\(l= 0\)

*and*\(k\le \min \{\Delta ,\lambda +\mu (1- \Theta (\infty ))\}\),

*where*

*and*

Recently, there were a number of papers focused on the Malmquist-type theorem of the complex difference equations. Ablowitz et al. (2000) proved some results on the classical Malmquist-type theorem of the complex difference equations by applying Nevanlinna theory. Besides, Gao, Xu and Li also studied some systems of complex difference equation and obtained some more precise results related to Malmquist-type theorem (see Gao 2012a, b, c; Li and Gao 2015; Xu et al. 2013, 2015; Xu and Xuan 2015). In this paper, we mainly study the *q*-difference differential equation about the Maimquist-type theorem, and obtain the following theorem.

###
**Theorem 11**

*Let*

*where*

*R*(

*z*,

*w*)

*is defined as*

*P*(

*z*,

*w*)

*and*

*Q*(

*z*,

*w*)

*are irreducible polynomials in*

*w*,

*coefficients*\(a_i(z),b_j(z)\)

*are rational functions of*

*z*.

*If*

*Eq.*(7)

*exists transcendental meromorphic solutions*

*with zero order, then we also think that*\(l= 0\)

*and*\(k\le 2n\).

Similar to the proof of Theorem 11, we can get the following corollary easily.

###
**Corollary 12**

*Let*

*where*

*R*(

*z*,

*w*)

*is defined as in Theorem*11.

*If Eq.*(8)

*has transcendental meromorphic solution of zero order*,

*then there will be*\(l= 0\)

*and*\(k\le \min \{\Delta ,\lambda +\mu (1- \Theta (\infty ))\}\),

*where*\(\Delta , \lambda\)

*and*\(\mu\)

*are stated as in Theorem*10.

## Some Lemmas

###
**Lemma 13**

*f*(

*z*)

*be a meromorphic function. Then for all irreducible rational functions in*

*f*,

*with meromorphic coefficients*\(a_i(z),b_j(z)\),

*the characteristic function of*

*R*(

*z*,

*f*(

*z*))

*satisfies*

*where*\(d=\max \{m,n\}\)

*and*\(\Psi (r)=\max _{i,j}\{T(r,a_i),T(r,b_j)\}\).

###
**Lemma 14**

*Let*

*f*(

*z*)

*be a transcendental meromorphic function of zero order and q be a nonzero complex constant. Then*

*and*

*on a set of logarithmic density 1*.

###
**Lemma 15**

*Let*

*f*(

*z*)

*be a nonconstant zero-order meromorphic function and*\(q \in \mathbb {C}{\setminus } \{0\}\).

*Then*

*on a set of logarithmic density 1 for all r outside a possible exceptional set of logarithmic density 0.*

###
**Lemma 16**

###
**Lemma 17**

*Let*\(q\in \mathbb {C}{\setminus }\{0\}\)

*and*

*f*(

*z*)

*be a nonconstant meromorphic function with zero order. Then for any positive finite integer*

*k*,

*we have*

*and*

###
*Proof*

## The Proof of Theorem 6

## The Proof of Theorem 11

Suppose that *w*(*z*) is a transcendental meromorphic solution of equation (7) with zero order, then \(\varphi (z)= \frac{1}{w(z)-a}\) is also a transcendental meromorphic solution of Eq. (17). We will discuss two cases as follows.

This completes the proof of Theorem 11.

## Declarations

### Authors' contributions

XLW, HW and HYX completed the main part of this article. All authors read and approved the final manuscript.

### Acknowledgements

The authors were supported by the NSF of China (11561033, 11301233), the Natural Science Foundation of Jiangxi Province in China (20151BAB201008), and the Foundation of Education Department of Jiangxi (GJJ150902) of China.

### Competing interests

The authors declare that they have no competing interests.

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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