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Growth of solutions to two systems of q-difference differential equations
Advances in Difference Equations volume 2020, Article number: 112 (2020)
Abstract
This paper is devoted to studying the growth of entire or meromorphic solutions to two systems of q-difference differential equations. The estimates on the growth order of meromorphic solutions are obtained, which are extensions of previous results due to Xu et al . Examples are given to illustrate the existence of solutions of such systems.
1 Introduction and main results
Let \(f(z)\) be a non-constant meromorphic function in the complex plane \(\mathbb{C}\). We use \(\rho(f)\) and \(\mu(f)\) to denote the order and the lower order of \(f(z)\), and use \(\lambda(\frac{1}{f})\) and \(\overline{\lambda}(\frac{1}{f})\) to denote the exponent of convergence of poles and that of the distinct poles of \(f(z)\), respectively. In addition, we say a meromorphic function \(\alpha(z)\) (\(\not\equiv0,\infty\)) is a small function of \(f(z)\) provided that \(T(r,\alpha)=S(r,f)\), where \(S(r,f)\) denotes any quantity that satisfies the condition \(S(r,f)=o(T(r,f))\) as \(r\rightarrow\infty\), possibly outside a set of r with finite logarithmic measure. Nevanlinna theory is an important tool in this paper, its standard symbols and fundamental results come mainly from [12, 21].
As we all know, it is an interesting problem to consider the Malmquist theorem [16] for differential equations. Laine [15] gave the following result.
Theorem A
([15])
Let
where the right-hand side
is rational in both arguments. If equation (1) has a transcendental meromorphic solution, then\(l=0\)and\(k\leq2n\).
With the establishment of the difference analog of Nevanlinna theory, many studies [1, 8, 13, 20] about the Malmquist-type theorem of complex difference equations or systems have appeared. Gundersen et al. [11] considered the growth of meromorphic solutions to a certain type of complex q-difference equation and proved the following result.
Theorem B
([11])
Let\(w(z)\)be a transcendental meromorphic solution of the equation
where\(q\in\mathbb{C}\), \(|q|>1\), \(R(z,w)\)is irreducible inw, which is defined as in Theorem A, and the coefficients\(a_{i}(z)\)and\(b_{j}(z)\)are small functions ofwand\(a_{k}(z)b_{l}(z)\not\equiv0\). If\(m: =\max\{k,l\}\geq1\), then\(\rho(w)=\frac{\log m}{\log|q|}\).
After these results, many scholars studied a series of complex q-difference differential equations and systems about the Malmquist-type theorem [5, 6, 19]. Xu et al. [19] investigated the following system:
where \(q_{1}, q_{2}\in\mathbb{C}\setminus\{0\}\), \(n_{1}, n_{2}\in{\mathbb {Z}_{+}}\), and
are irreducible rational functions, and \(a_{i}(z)\), \(b_{j}(z)\) are small functions with respect to \(w_{1}\), and \(c_{i}(z)\), \(d_{j}(z)\) are small functions with respect to \(w_{2}\). They obtained the estimates on the growth order for meromorphic solutions of system (2).
We consider the question of what happens if system (2) is more general, for example,
where \(q_{1}, q_{2}\in\mathbb{C}\setminus\{0\}\), \(h_{1}, h_{2}\in{\mathbb{Z}_{+}}\), and \(R_{1}(z,w_{1}(z))\), \(R_{2}(z,w_{2}(z))\) are defined as in (3), and
are irreducible rational functions in \(w_{1}^{(h_{1})}(q_{1}z)\), \(w_{2}^{(h_{2})}(q_{2}z)\), respectively, and the meromorphic coefficients \(u_{m_{t}}^{t}(z)\) (\(m_{t}=0,\ldots,p_{t}\)), \(v_{n_{t}}^{t}(z)\) (\(n_{t}=0,\ldots,s_{t}\)) are of growth \(S(r,w_{t})\), \(t=1,2\), and \(u_{p_{t}}^{t}(z)v_{s_{t}}^{t}(z)\not\equiv0\), \(t=1,2\).
For the question above, we study the growth of solutions to the system of q-difference differential equations (4). Further, set
Clearly, \(\tau_{t}\geq1\) and \(\sigma_{t}\geq1\). Also set
and
Now, we state the first result in this paper.
Theorem 1.1
Let\((w_{1},w_{2})\)be a pair of transcendental solutions of system (4). Then one of the following cases holds.
- (i)
For\(|q_{1}|>1\), \(|q_{2}|>1\), if\(w_{1}\), \(w_{2}\)are meromorphic and\(\sigma>\kappa \), then\(\mu(w_{t})\geq\frac{\log\sigma-\log\kappa}{\log|q|}\), \(t=1,2\); if\(w_{t}\)is meromorphic and\(\sigma>\kappa_{t}\), \(t=1\)or\(t=2\), and the other is entire, then\(\mu(w_{t})\geq\frac{\log\sigma-\log\kappa_{t}}{\log|q|}\), \(t=1,2\); if\(w_{1}\), \(w_{2}\)are entire and\(\sigma>\tau\), then\(\mu(w_{t})\geq\frac{\log\sigma-\log\tau}{\log|q|}\), \(t=1,2\).
- (ii)
For\(|q_{1}|<1\), \(|q_{2}|<1\), if\(w_{1}\), \(w_{2}\)are meromorphic and\(\sigma\leq\kappa\), then\(\rho(w_{t})\leq\frac{\log\sigma-\log\kappa}{\log|q|}\), \(t=1,2\); if\(w_{t}\)is meromorphic and\(\sigma\leq\kappa_{t}\), \(t=1\)or\(t=2\), and the other is entire, then\(\rho(w_{t})\leq\frac{\log\sigma-\log\kappa_{t}}{\log|q|}\), \(t=1,2\); if\(w_{1}\), \(w_{2}\)are entire and\(\sigma\leq\tau\), then\(\rho(w_{t})\leq\frac{\log\sigma-\log\tau}{\log|q|}\), \(t=1,2\).
- (iii)
For\(|q_{1}|=|q_{2}|=1\), if\(w_{1}\), \(w_{2}\)are meromorphic, then\(\sigma\leq\kappa\), furthermore, if\(\kappa_{t}<\sigma\leq\kappa\), \(t=1,2\), then\(\overline{\lambda} (\frac{1}{w_{t}} )=\lambda (\frac {1}{w_{t}} )=\rho(w_{t})\), \(t=1,2\); if\(w_{t}\)is meromorphic, \(t=1\)or\(t=2\), and the other is entire, then\(\sigma\leq\kappa_{t}\), \(t=1\)or\(t=2\), furthermore, if\(\tau<\sigma\leq\kappa_{t}\), \(t=1\)or\(t=2\), then\(\overline{\lambda} (\frac{1}{w_{t}} )=\lambda (\frac {1}{w_{t}} )=\rho(w_{t})\), \(t=1\)or\(t=2\); if\(w_{1}\), \(w_{2}\)are entire, then\(\sigma\leq\tau\).
In the past few decades, meromorphic solutions of complex functional equations were studied by Bergweiler et al. [3, 4], Heittokangas et al. [14], and Rieppo [17]. Silvennoinen [18] investigated the existence and growth of solutions to an equation of the form \(w(g(z))=R(z,w)\) and proved the following result.
Theorem C
([18])
Let
where the right-hand side\(R(z,w)\)is defined as in Theorem A, \(a_{i}(z)\), \(b_{j}(z)\)are of growth\(S(r,w)\), and\(g(z)\)is entire. If equation (6) has a non-constant meromorphic solutionw, then\(g(z)\)is a polynomial.
Gao [7] considered the system of functional equations
where \(g(z)\) is an entire function, \(R_{1}(z,w_{1}(z))\), \(R_{2}(z,w_{2}(z))\) are defined as in (3), and obtained the following result.
Theorem D
([7])
If system (7) has a pair of non-constant meromorphic solutions\((w_{1},w_{2})\), then\(g(z)\)is a polynomial.
There are some results about the existence and growth of meromorphic solutions of several systems of complex functional equations [8, 19, 20]. Xu et al. [19] studied the problem when \(R_{1}(z,w_{1}(z))\), \(R_{2}(z,w_{2}(z))\) in (2) are replaced by \(R_{1}(z,w_{1}(g_{1}(z)))\), \(R_{2}(z,w_{2}(g_{2}(z)))\), respectively, and (2) is turned into the following system:
where \(g_{1}(z)\), \(g_{2}(z)\) are polynomials, and obtained the estimates of the growth order of meromorphic solutions of system (8).
A similar question to ask is what happens if system (8) is more general, for example,
where \(R_{1}(z,w_{1}(z))\), \(R_{2}(z,w_{2}(z))\), \(\varOmega_{1} (z,w_{1}^{(h_{1})}(q_{1}z) )\), and \(\varOmega_{2} (z,w_{2}^{(h_{2})}(q_{2}z) )\) are defined as in (3), (5), respectively. Further, set
and
be two polynomials, where \(\alpha_{\gamma_{1}},\alpha_{\gamma_{1}-1},\ldots ,\alpha_{0}\), \(\beta_{\gamma_{2}},\beta_{\gamma_{2}-1},\ldots,\beta_{0}\) are complex constants, and \(\gamma_{t}\geq2\) (\(t=1,2\)) are two positive integers.
The second result in this paper concerns the growth of solutions to the system of functional equations (9).
Theorem 1.2
Let\((w_{1},w_{2})\)be a pair of transcendental solutions of system (9). Then one of the following cases holds.
- (i)
If\(w_{1}\), \(w_{2}\)are meromorphic and\(\sigma\leq\kappa\), then
$$\begin{aligned} T \bigl(r,w_{t}(z) \bigr)=O \bigl((\log r)^{\alpha_{1}} \bigr), \quad t=1,2, \end{aligned}$$where\(\alpha_{1}=\frac{\log\kappa-\log\sigma}{\log(\gamma_{1}\gamma_{2})}\).
- (ii)
If\(w_{t}\)is meromorphic and\(\sigma\leq\kappa_{t}\), \(t=1\)or\(t=2\), and the other is entire, then
$$\begin{aligned} T \bigl(r,w_{t}(z) \bigr)=O \bigl((\log r)^{\alpha_{2}} \bigr), \quad t=1 \textit{ or } t=2, \end{aligned}$$where\(\alpha_{2}=\frac{\log\kappa_{t}-\log\sigma}{\log(\gamma_{1}\gamma _{2})}\), \(t=1\)or\(t=2\).
- (iii)
If\(w_{1}\), \(w_{2}\)are entire and\(\sigma\leq\tau\), then
$$\begin{aligned} T \bigl(r,w_{t}(z) \bigr)=O \bigl((\log r)^{\alpha_{3}} \bigr), \quad t=1,2, \end{aligned}$$where\(\alpha_{3}=\frac{\log\tau-\log\sigma}{\log(\gamma_{1}\gamma_{2})}\).
2 Examples
In this section, we give examples to illustrate that the cases can occur in Theorem 1.1.
The following Examples 2.1–2.4 are about case (i) of Theorem 1.1.
Example 2.1
Let \(q_{1}=2\), \(q_{2}=3\). Then \((w_{1},w_{2})= (\frac{e^{z}}{z},\frac {e^{z}}{z^{2}} )\) satisfies the system
where \(h_{1}=h_{2}=1\), \(\tau_{1}=3\), \(\tau_{2}=2\), and \(\sigma_{1}=\sigma_{2}=6\). Here \(\sigma=36>\kappa=24\) and \(\mu(w_{t})=1\geq\frac{\log36-\log24}{\log6}=\frac{\log36-\log24}{\log 36-\log6}\), \(t=1,2\).
Example 2.2
Let \(q_{1}=2\), \(q_{2}=3\). Then \((w_{1},w_{2})= (e^{z},\frac{e^{z}}{z} )\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=3\), \(\tau_{2}=2\), and \(\sigma_{1}=\sigma_{2}=6\). Then we have \(\sigma=36>\kappa_{2}=12\) and \(\mu(w_{t})=1\geq\frac{\log36-\log12}{\log6}=\frac{\log3}{\log6}\), \(t=1,2\).
Example 2.3
Let \(q_{1}=3\), \(q_{2}=2\). Then \((w_{1},w_{2})= (\frac{e^{z}}{z^{2}},e^{z} )\) satisfies the system
where \(h_{1}=1\), \(h_{2}=2\), \(\tau_{1}=3\), \(\tau_{2}=2\), \(\sigma_{1}=4\), and \(\sigma_{2}=6\). It is known that \(\sigma=24>\kappa_{1}=12\) and \(\mu(w_{t})=1\geq\frac{\log24-\log12}{\log6}=\frac{\log2}{\log6}\), \(t=1,2\).
Example 2.4
Let \(q_{1}=2\), \(q_{2}=3\). Then \((w_{1},w_{2})=(e^{z},ze^{z})\) satisfies the system
where \(h_{1}=h_{2}=2\), \(\tau_{1}=3\), \(\tau_{2}=2\), and \(\sigma_{1}=\sigma_{2}=6\). Thus, \(\sigma=36>\tau=6\) and \(\mu(w_{t})=1\geq\frac{\log36-\log6}{\log6}=1\), \(t=1,2\).
The following Examples 2.5–2.8 are about case (ii) of Theorem 1.1.
Example 2.5
Let \(q_{1}=\frac{1}{2}\), \(q_{2}=\frac{1}{3}\). Then \((w_{1},w_{2})= (\frac {e^{z}}{z},\frac{e^{z}}{z-1} )\) satisfies the system
where \(h_{1}=h_{2}=1\), \(\tau_{1}=4\), \(\tau_{2}=6\), and \(\sigma_{1}=\sigma_{2}=2\). Clearly, \(\sigma=4\leq\kappa=96\) and \(\rho(w_{t})=1\leq\frac{\log4-\log96}{\log\frac{1}{6}}=\frac{\log24}{\log 6}\), \(t=1,2\).
Example 2.6
Let \(q_{1}=\frac{1}{3}\), \(q_{2}=\frac{1}{2}\). Then \((w_{1},w_{2})= (e^{z},\frac {e^{z}}{z-1} )\) satisfies the system
where \(h_{1}=h_{2}=1\), \(\tau_{1}=6\), \(\tau_{2}=4\), and \(\sigma_{1}=\sigma_{2}=2\). Then we have \(\sigma=4\leq\kappa_{2}=48\) and \(\rho(w_{t})=1\leq\frac{\log4-\log48}{\log\frac{1}{6}}=\frac{\log12}{\log 6}\), \(t=1,2\).
Example 2.7
Let \(q_{1}=\frac{1}{2}\), \(q_{2}=\frac{1}{4}\). Then \((w_{1},w_{2})= (\frac {ze^{z}}{z-1},ze^{z} )\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=2\), \(\tau_{2}=8\), \(\sigma_{1}=2\), and \(\sigma_{2}=1\). It is known that \(\sigma=2\leq\kappa_{1}=48\) and \(\rho(w_{t})=1\leq\frac{\log2-\log48}{\log\frac{1}{8}}=\frac{\log24}{\log 8}\), \(t=1,2\).
Example 2.8
Let \(q_{1}=\frac{1}{2}\), \(q_{2}=\frac{1}{4}\). Then \((w_{1},w_{2})=(ze^{z},e^{2z})\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=4\), \(\tau_{2}=2\), and \(\sigma_{1}=\sigma_{2}=1\). Here \(\sigma=1\leq\tau=8\) and \(\rho(w_{t})=1\leq\frac{\log1-\log8}{\log\frac{1}{8}}=1\), \(t=1,2\).
The following Examples 2.9–2.12 are about case (iii) of Theorem 1.1.
Example 2.9
Let \(q_{1}=q_{2}=1\). Then \((w_{1},w_{2})= (\frac{e^{z}}{e^{z}-1},\frac {ze^{z}}{e^{z}-1} )\) satisfies the system
where \(h_{1}=h_{2}=1\), \(\tau_{1}=2\), \(\tau_{2}=1\), and \(\sigma_{1}=\sigma_{2}=2\). Clearly, \(\sigma=4\leq\kappa=8\).
Example 2.10
Let \(q_{1}=1\), \(q_{2}=-1\). Then \((w_{1},w_{2})= (\frac{1}{e^{z}-1},\frac {1}{1-e^{z}} )\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=\tau_{2}=1\), \(\sigma_{1}=2\), and \(\sigma_{2}=3\). Then we have \(\kappa_{t}<\sigma=6\leq\kappa=6\), \(t=1,2\), and \(\overline{\lambda} (\frac{1}{w_{t}} )=\lambda (\frac {1}{w_{t}} )=\rho(w_{t})=1\), \(t=1,2\).
Example 2.11
Let \(q_{1}=q_{2}=1\). Then \((w_{1},w_{2})= (e^{z},\frac{1}{e^{z}-1} )\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=2\), \(\tau_{2}=4\), \(\sigma_{1}=8\), and \(\sigma_{2}=2\). It is known that \(\tau=8<\sigma=16\leq\kappa_{2}=16\) and \(\overline{\lambda} (\frac{1}{w_{2}} )=\lambda (\frac {1}{w_{2}} )=\rho(w_{2})=1\).
Example 2.12
Let \(q_{1}=q_{2}=1\). Then \((w_{1},w_{2})=(e^{z},ze^{2z})\) satisfies the system
where \(h_{1}=2\), \(h_{2}=1\), \(\tau_{1}=4\), \(\tau_{2}=2\), \(\sigma_{1}=4\), and \(\sigma_{2}=1\). Then we have \(\sigma=4\leq\tau=8\).
3 Lemmas
To prove Theorems 1.1 and 1.2, we need the following lemmas. Yang and Yi [21] showed the value distribution of a meromorphic function and its derivative.
Lemma 3.1
([21])
The following lemma is to compare the Nevanlinna functions of \(f(z)\) and \(f(cz)\).
Lemma 3.2
([3])
In 1972, Bank [2] established the following lemma.
Lemma 3.3
([2])
Let\(g(r)\)and\(h(r)\)be monotone non-decreasing functions on\((0,+\infty)\)such that\(g(r)\leq h(r)\), possibly outside a set ofrwith finite logarithmic measure. Then, for any real number\(a>1\), there exists\(r_{0}>0\)such that\(g(r)\leq h(ar)\)for all\(r>r_{0}\).
Gundersen et al. [11] showed a method to obtain an upper bound for the growth order of a meromorphic function.
Lemma 3.4
([11])
Let\(f(z)\)be a non-constant meromorphic function, and let\(\varPsi: (1,\infty)\rightarrow(0,\infty)\)be a monotone non-decreasing function. If for some real number\(a\in(0,1)\), there exist real numbers\(K_{1}>0\)and\(K_{2}\geq1\)such that
then
The following lemma gives us a method to have a lower bound for the lower order of a meromorphic function.
Lemma 3.5
([17])
Let\(\varPsi: (r_{0},\infty)\rightarrow(1,\infty)\)be a monotone non-decreasing function, where\(r_{0}\geq1\). If for some real number\(a>1\), there exists a real number\(b>1\)such that\(\varPsi(ar)\geq b\varPsi(r)\), then
The following result about estimate of the Nevanlinna characteristic function of a meromorphic function composed with polynomials is given by Goldstein.
Lemma 3.6
([9])
Let\(f(z)\)be a transcendental meromorphic function and\(g(z)=a_{m}z^{m}+a_{m-1}z^{m-1}+\cdots+a_{0}\)be a polynomial with degreem (≥1). For given\(\delta\in(0,|a_{m}|)\), let\(\lambda=|a_{m}|+\delta\), \(\mu =|a_{m}|-\delta\), then
for any given\(\varepsilon>0\)and sufficiently larger.
Goldstein [10] showed the following lemma.
Lemma 3.7
([10])
Let\(\phi(r)\)be a positive function defined on\([r_{0},\infty)\)and bounded in every finite interval. Assume that\(\phi(\mu r^{k})\leq a\phi(r)+b\) (\(r\geq r_{0}\)), whereμ (>0), k (>1), a (≥1), andbare constants. Then\(\phi(r)=O((\log r)^{\alpha})\)with\(\alpha =\frac{\log a}{\log k}\), unless\(a=1\)and\(b>0\); and if\(a=1\)and\(b>0\), then for any\(\varepsilon>0\), \(\phi(r)=O((\log r)^{\varepsilon})\).
4 Proofs of the results
Proof of Theorem 1.1
Suppose first that \((w_{1},w_{2})\) is a pair of transcendental solutions of system (4). In the following, we consider three cases.
Case (i): \(|q_{1}|>1\) and \(|q_{2}|>1\). Suppose that both \(w_{1}\) and \(w_{2}\) are meromorphic. It follows from Valiron–Mohon’ko theorem [15, Theorem 2.2.5], Lemma 3.1, and Lemma 3.2 that
that is,
Similarly, we have
Thus, from (10) and (11), we obtain
Now \(\sigma>\kappa\), and for any given \(\varepsilon>0\),
for sufficiently large r, possibly outside a set of r with finite logarithmic measure. By Lemma 3.3, with \(a>1\) and (13) we have
for all \(r\geq r_{0}\). It follows from Lemma 3.5 and (14) that
As \(\varepsilon\rightarrow0^{+}\) and \(a\rightarrow1^{+}\), we get
Suppose that only one between \(w_{1}\) and \(w_{2}\) is meromorphic, without loss of generality, we assume that \(w_{1}\) is meromorphic and \(w_{2}\) is entire. Then, similar to (11), we have
Thus, it follows from (10) and (15) that
Similar to the above argument, since \(\sigma>\kappa_{1}\) and for any small \(\varepsilon>0\), we know that there exists \(a>1\) such that
for all \(r\geq r_{0}\). Applying Lemma 3.5 to (17) yields that
By letting \(\varepsilon\rightarrow0^{+}\) and \(a\rightarrow1^{+}\), we obtain
Suppose that both \(w_{1}\) and \(w_{2}\) are entire. Then, similar to (10), we have
Thus, it follows from (15) and (18) that
Now, \(\sigma>\tau\), we know that for \(\varepsilon>0\) there exists \(a>1\) such that
for all \(r\geq r_{0}\). Recalling Lemma 3.5 and letting \(\varepsilon\rightarrow0^{+}\) and \(a\rightarrow1^{+}\), we conclude that
Case (ii): \(|q_{1}|<1\) and \(|q_{2}|<1\). Suppose that both \(w_{1}\) and \(w_{2}\) are meromorphic. Then, similar to the previous argument, we have that for \(\varepsilon>0\) there exists \(a>1\) such that \(a|q|<1\), (12) and (14) hold for all \(r\geq r_{0}\). Since \(\sigma\leq\kappa\), then \(\frac{\kappa(1+\varepsilon)}{\sigma(1-\varepsilon)}>1\). Hence, applying Lemma 3.4 to (14) yields that
which implies
as \(\varepsilon\rightarrow0^{+}\) and \(a\rightarrow1^{+}\).
Suppose that only one between \(w_{1}\) and \(w_{2}\) is meromorphic. Without loss of generality, we assume that \(w_{1}\) is meromorphic and \(w_{2}\) is entire. Then we similarly obtain that, for \(\varepsilon>0\), there exists \(a>1\) such that \(a|q|<1\), (16) and (17) hold for all \(r\geq r_{0}\). Since \(\sigma\leq\kappa_{1}\), then \(\frac{\kappa_{1}(1+\varepsilon)}{\sigma(1-\varepsilon)}>1\). Thus, we conclude by Lemma 3.4 and (17) that
and let \(\varepsilon\rightarrow0^{+}\) and \(\alpha\rightarrow1^{+}\), it yields
Suppose that both \(w_{1}\) and \(w_{2}\) are entire. Similarly, for \(\varepsilon>0\), there exists \(a>1\) such that \(a|q|<1\), (15), (18), and (19) hold for all \(r\geq r_{0}\). Since \(\sigma\leq\tau\), then \(\frac{\tau(1+\varepsilon)}{\sigma(1-\varepsilon)}>1\). Therefore, recalling Lemma 3.4, we have
which deduces
as \(\varepsilon\rightarrow0^{+}\) and \(a\rightarrow1^{+}\).
Case (iii): \(|q_{1}|=|q_{2}|=1\). Suppose that both \(w_{1}\) and \(w_{2}\) are meromorphic. Then, from Valiron–Mohon’ko theorem [15, Theorem 2.2.5] and Lemma 3.1, we conclude that
and
From (20) and (21), we have \(\sigma\leq\kappa\). Furthermore, if \(\kappa_{t}<\sigma\leq\kappa\), \(t=1,2\), then
and
which imply that \(\overline{\lambda} (\frac{1}{w_{t}} )=\lambda (\frac{1}{w_{t}} )=\rho(w_{t})\), \(t=1,2\).
Suppose that only one between \(w_{1}\) and \(w_{2}\) is meromorphic. Without loss of generality, we assume that \(w_{1}\) is meromorphic and \(w_{2}\) is entire. Then we get (20) and
Hence, it follows from (20) and (22) that \(\sigma\leq \kappa_{1}\). Furthermore, if \(\tau<\sigma\leq\kappa_{1}\), it yields
which implies \(\overline{\lambda} (\frac{1}{w_{1}} )=\lambda (\frac{1}{w_{1}} )=\rho(w_{1})\). Similarly, if \(w_{2}\) is meromorphic and \(w_{1}\) is entire, we obtain that \(\overline{\lambda} (\frac{1}{w_{2}} )=\lambda (\frac {1}{w_{2}} )=\rho(w_{2})\) when \(\tau<\sigma\leq\kappa_{2}\).
Suppose that both \(w_{1}\) and \(w_{2}\) are entire. Then, similar to the above argument, we can get (22) and
Thus, it follows from (22) and (23) that \(\sigma\leq\tau\).
From Cases (i)–(iii), the proof of Theorem 1.1 is completed. □
Proof of Theorem 1.2
Suppose first that \((w_{1},w_{2})\) is a pair of transcendental solutions of system (9). In what follows, we consider three cases.
Case (i): Suppose that both \(w_{1}\) and \(w_{2}\) are meromorphic. Then, by Valiron–Mohon’ko theorem [15, Theorem 2.2.5], Lemma 3.1, and Lemma 3.2, we get
and
By Lemma 3.6, for given \(0<\delta_{1}<|\alpha_{\gamma_{1}}|\), \(0<\delta_{2}<|\beta_{\gamma_{2}}|\), and \(\mu_{1}=|\alpha_{\gamma_{1}}|-\delta_{1}\), \(\mu_{2}=|\beta_{\gamma _{2}}|-\delta_{2}\), we know that for any small \(\varepsilon>0\) there exists two sets \(E_{1}\), \(E_{2}\) of finite logarithmic measure such that
and
Thus, for sufficiently large r and \(r\notin E_{1}\cup E_{2}\), we can deduce from (26) and (27) that
and
By Lemma 3.3, with \(a>1\) and (28), we have
for all \(r\geq r_{0}\). Set \(R=a|q_{1}|r\). Then (30) can be rewritten as
If \(\sigma\leq\kappa\), then \(\frac{\kappa(1+\varepsilon)^{2}}{\sigma(1-\varepsilon)^{2}}\geq1\). Since \(\frac{\mu_{1}\mu_{2}^{\gamma_{1}}}{|q_{2}|^{\gamma_{1}}|aq_{1}|^{\gamma _{1}\gamma_{2}}}>0\), \(\gamma_{t}\geq2\) (\(t=1,2\)), applying Lemma 3.7 to (31) yields that
where
which deduces
as \(\varepsilon\rightarrow0^{+}\). Similarly, from (29), we conclude that
where
Case (ii): Suppose that only one between \(w_{1}\) and \(w_{2}\) is meromorphic. Without loss of generality, we assume that \(w_{2}\) is meromorphic and \(w_{1}\) is entire. By Valiron–Mohon’ko theorem [15, Theorem 2.2.5], Lemma 3.1, and Lemma 3.2, we get (24) and
Thus, by an argument similar to the proof of Case (i) of Theorem 1.2, we can deduce
If \(\sigma\leq\kappa_{2}\), then \(\frac{\kappa_{2}(1+\varepsilon)^{2}}{\sigma(1-\varepsilon)^{2}}\geq1\). Since \(\frac{\mu_{1}\mu_{2}^{\gamma_{1}}}{|q_{2}|^{\gamma_{1}}|aq_{1}|^{\gamma _{1}\gamma_{2}}}>0\), \(\gamma_{t}\geq2\) (\(t=1,2\)), it follows from Lemma 3.7 that
where
Similarly, we have
where
Case (iii): Suppose that both \(w_{1}\) and \(w_{2}\) are entire. Then, similar to the above argument, we can get (32) and
Hence, by an argument similar to the proof of Case (ii) of Theorem 1.2, if \(\sigma\leq\tau\), then we can obtain
where
From Cases (i)–(iii), the proof of Theorem 1.2 is completed. □
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Tu, H., Yuan, W. Growth of solutions to two systems of q-difference differential equations. Adv Differ Equ 2020, 112 (2020). https://doi.org/10.1186/s13662-020-02565-5
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DOI: https://doi.org/10.1186/s13662-020-02565-5