 Research
 Open Access
 Published:
On the hyperorder of solutions of two class of complex linear differential equations
Advances in Difference Equations volume 2015, Article number: 234 (2015)
Abstract
We investigate the hyperorder of solutions of two class of complex linear differential equations. We investigate the growth of solutions of higher order and certain second order linear differential equations, and we obtain some results which improve and extend some previous results in complex oscillations.
Introduction and results
We shall assume that reader is familiar with the fundamental results and the standard notations of the Nevanlinna value distribution theory of meromorphic functions (e.g. see [1, 2]). In this paper, we use \(\rho(f)\), \(\tau(f)\) to denote the order and type of an entire function \(f(z)\), use \(\lambda(f)\) (\(\overline{\lambda}(f)\)) to denote the exponent of convergence of zeros (distinct zeros) of \(f(z)\), and use \(\rho_{2}(f)\) to denote the hyperorder of \(f(z)\) (see [3]), which is defined to be
The hyperexponent of convergence of zeros and distinct zeros of \(f(z)\) are, respectively, defined to be (see [4])
In addition, we use M to denote a positive constant, not necessarily the same at each occurrence. We denote the linear measure of a set \(E\subset(0,+\infty)\) by \(mE=\int_{E}dt\) and the logarithmic measure of E by \(m_{l}E=\int_{E}dt/t\), respectively. The upper and the lower logarithmic density of E are defined by
For the second order linear differential equation
where \(A(z)\) and \(B(z)\not\equiv0\) are entire functions of finite order, it is well known that every solution \(f\not\equiv0\) of (1.1) is of infinite order if \(\rho(A)<\rho(B)\) or \(\rho(B)<\rho(A)\leq1/2\) (see [5–7]). For the case of \(\rho(A)>1/2\) and \(\rho(B)<\rho(A)\), the possibility of solutions of infinite order of (1.1) remains open, many authors have studied the problem (e.g. see [8–11]). In 2000, Laine and Wu proved the following.
Theorem A
(see [10])
Suppose that \(\rho(B)<\rho(A)<\infty\) and that \(T(r,A)\sim\log M(r,A)\) as \(r\rightarrow\infty\) outside a set of finite logarithmic measure. Then every nonconstant solution f of (1.1) is of infinite order.
For the higher order linear differential equation
there are similar results as follows.
Theorem B
(see [12])
Let \(A_{j}(z)\) (\(j=0,\ldots,k1\)), \(F(z)\not\equiv0\) be entire functions. Suppose that there exists some \(d\in\{1,\ldots,k1\}\) such that \(\max\{\rho(F), \rho(A_{j}): j\neq d\}=\rho<\rho(A_{d})<\infty\) and \(T(r,A_{d})\sim\log M(r,A_{d})\) as \(r\rightarrow\infty\) outside a set of upper logarithmic density less than \((\rho(A_{d})\rho)/\rho(A_{d})\). Then every transcendental solution \(f(z)\) of (1.2) satisfies \(\overline{\lambda}(f)=\lambda(f)=\rho(f)=\infty\).
Theorem C
(see [12])
Let \(A_{j}(z)\) (\(j=0,\ldots,k1\)), \(F(z)\equiv0\) be entire functions. Suppose that there exists some \(d\in\{1,\ldots,k1\}\) such that \(\max\{\rho(A_{j}): j\neq0,d\}<\rho(A_{0})\leq\frac{1}{2}\) and that \(A_{d}(z)\) has a finite deficient value. Then every solution \(f(z)\not\equiv0\) of (1.2) satisfies \(\rho(A_{0})\leq\rho_{2}(f)\leq\rho(A_{d})\).
Then a natural question is: Can we estimate the hyperorder of the solutions of (1.1) and (1.2) under the same condition in Theorems A and B? And: Can we estimate the hyperorder of the solutions of (1.2) in Theorem C if \(\rho(A_{0})>\frac{1}{2}\)? Theorems 1.1 and 1.2 below give answers to the above questions.
At the same time, many authors have investigated the growth of solutions of (1.1) and its nonhomogeneous linear differential equation
when \(\rho(A)=\rho(B)\) and obtained the following results.
Theorem D
(see [8])
Let \(P(z)\) and \(Q(z)\) be nonconstant polynomials such that \(P(z)=a_{n}z^{n}+a_{n1}z^{n1}+\cdots +a_{1}z+a_{0}\), \(Q(z)=b_{n}z^{n}+b_{n1}z^{n1}+\cdots+b_{1}z+b_{0}\) for some complex numbers \(a_{i}\), \(b_{i}\) (\(i=0,\ldots,n\)) with \(a_{n}b_{n}\neq0\) and let \(A_{1}(z)\) and \(A_{0}(z)\not\equiv0\) be entire functions satisfying \(\rho(A_{1})< n\) and \(\rho(A_{0})< n\). Then the following statements hold:

(i)
If either \(\arg a_{n}\neq \arg b_{n}\) or \(a_{n}=cb_{n}\) with \(0< c<1\), then every nonconstant solution f of
$$ f''+A_{1}(z)e^{P(z)}f'+A_{0}(z)e^{Q(z)}f=0 $$(1.4)has infinite order with \(\rho_{2}(f)\geq n\).

(ii)
Let \(a_{n}=b_{n}\) and \(\operatorname{deg}(PQ)=m\geq 1\), and let the orders of \(A_{1}(z)\) and \(A_{0}(z)\) be less than m. Then every nonconstant solution f of (1.4) has infinite order with \(\rho_{2}(f)\geq m\).

(iii)
Let \(a_{n}=cb_{n}\) with \(c>1\) and \(\operatorname{deg}(PcQ)=m\geq1\). Suppose \(\rho(A_{1})< m\) and \(0<\rho(A_{0})<\frac{1}{2}\). Then every nonconstant solution of (1.4) has infinite order with \(\rho_{2}(f)\geq\rho(A_{0})\).

(iv)
Let \(a_{n}=cb_{n}\) with \(c>1\) and let \(PcQ\) be a constant. Suppose that \(\rho(A_{1})<\rho(A_{0})<\frac{1}{2}\). Then every nonconstant solution of (1.4) has infinite order with \(\rho_{2}(f)\geq\rho(A_{0})\).
Theorem E
(see [13])
Let a, b be nonzero complex numbers and \(a\neq b\), \(Q(z)\) be a nonconstant polynomial or \(Q(z)=h(z)e^{bz}\), where \(h(z)\not\equiv0\) is a polynomial. Then every solution \(f\not\equiv0\) of the equation
has infinite order and \(\rho_{2}(f)=1\).
Theorem F
(see [14])
Suppose that \(A_{0}\not\equiv0\), \(A_{1}\not\equiv0\), F are entire functions of order less than one, and the complex constants a, b satisfy \(ab\neq0\) and \(b\neq a\). Then every nontrivial solution f of
is of infinite order.
Theorem G
(see [15])
Let \(P(z)=a_{n}z^{n}+\cdots+a_{0}\), \(Q(z)=b_{n}z^{n}+\cdots+b_{0}\) be polynomials of degree \(n\geq1\) where \(a_{i}\), \(b_{i}\) (\(i=0, 1, \ldots,n\)) are complex numbers, and let \(A_{0}(z)\not\equiv0\), \(A_{1}(z)\not\equiv0\), \(F(z)\) be entire functions with order less than n. If \(a_{n}\neq b_{n}\), then every solution \(f\not\equiv0\) of
is of infinite order. Furthermore, if \(F(z)\not\equiv0\), then every solution f of (1.7) satisfies \(\overline{\lambda}(f)=\lambda(f)=\rho(f)=\infty\).
Theorem D left us a question: Can we have \(\rho_{2}(f)=n\) (n is a positive integer) for every nontrivial solution of (1.4) if \(a_{n}\neq b_{n}\)? Theorem E tells us that the question holds if \(n=1\). Many authors investigated the above question but none of them solve the question completely, and Theorem 1.3 completely solves this question. In the following, we give our results.
Theorem 1.1
Let \(A_{j}\) (\(j=0,\ldots,k1\)), \(F(z)\) be entire functions. Suppose that there exists some \(d\in\{1,\ldots,k1\}\) such that \(\max\{\rho(A_{j}),\rho(F):j\neq d\}\leq\rho(A_{d})<\infty\), \(\max\{\tau (A_{j}):\rho(A_{j})=\rho(A_{d}),\tau(F)\}<\tau(A_{d})\) and that \(T(r,A_{d})\sim\log M(r,A_{d})\) as \(r\rightarrow\infty\) outside a set of r of finite logarithmic measure. Then we have:

(i)
Every transcendental solution f of (1.2) satisfies \(\rho_{2}(f)=\rho(A_{d})\), and (1.2) may have polynomial solutions f of degree <d.

(ii)
If \(F(z)\not\equiv0\), then every transcendental solution f of (1.2) satisfies \(\overline{\lambda}_{2}(f)=\lambda_{2}(f)=\rho_{2}(f)=\rho(A_{d})\).

(iii)
If \(d=1\), then every nonconstant solution f of (1.2) satisfies \(\rho_{2}(f)=\rho(A_{1})\). Furthermore, if \(F(z)\not\equiv 0\), then every nonconstant solution f of (1.2) satisfies \(\overline{\lambda}_{2}(f)=\lambda_{2}(f)=\rho_{2}(f)=\rho(A_{1})\).
Theorem 1.2
Let \(A_{j}\) (\(j=0,\ldots,k1\)), \(F(z)\equiv0\) be entire functions satisfying \(\max\{\rho(A_{j}): j\neq0,d\}<\rho(A_{0})<\rho(A_{d})<\infty\). Suppose that \(T(r,A_{0})\sim\log M(r,A_{0})\) as \(r\rightarrow\infty\) outside a set of r of finite logarithmic measure and that \(A_{d}\) has a finite deficient value. Then every solution \(f\not\equiv0\) of (1.2) satisfies \(\rho(A_{0})\leq\rho_{2}(f)\leq\rho (A_{d})\).
Theorem 1.3
Let \(P(z)\), \(Q(z)\), \(A_{0}(z)\), \(A_{1}(z)\), \(F(z)\) satisfy the hypotheses of Theorem G. Then we have:

(1)
If \(a_{n}\neq b_{n}\), \(F(z)\equiv0\), then every solution \(f\not\equiv0\) of (1.7) satisfies \(\rho_{2}(f)=n\).

(2)
If \(a_{n}=c b_{n}\) (\(c<0\)), \(F(z)\not\equiv 0\), then every solution f of (1.7) satisfies \(\overline{\lambda}_{2}(f)=\lambda_{2}(f)=\rho_{2}(f)=n\).
Remark 1.1
Theorems 1.1 and 1.2 are improvements of Theorems AC. Theorem 1.3 is an improvement of Theorems D, E and a supplement to Theorems F, G.
Lemmas
Lemma 2.1
(see [16], p.399)
Let \(A_{j}(z)\) (\(j=0,\ldots,k1\)), \(F(z)\) be entire functions satisfying \(\max\{\rho(A_{j}),\rho(F):j=0,\ldots,k1\}\leq\rho<\infty\). Then every solution f of (1.2) satisfies \(\rho_{2}(f)\leq\rho\).
Lemma 2.2
(see [17])
Let \(f(z)\) be a transcendental meromorphic function, and let \(\alpha>1\) be a given constant. Then for any given constant and for any given \(\varepsilon>0\):

(i)
There exist a constant \(B>0\) and a set \(E_{1}\subset(0,\infty)\) having finite logarithmic measure such that, for all z satisfying \(z=r\notin E_{1}\), we have
$$ \biggl\vert \frac{f^{(j)}(z)}{f^{(i)}(z)}\biggr\vert \leq B\biggl[\frac{T(\alpha r,f)}{r}(\log r)^{\alpha}\log T(\alpha r,f)\biggr]^{ji} \quad (0\leq i< j). $$(2.1) 
(ii)
There exist a set \(H_{1}\subset[0,2\pi)\) that has linear measure zero and a constant \(B>0\) that depends only on α, for any \(\theta\in[0,2\pi)\backslash H_{1}\), there exists a constant \(R_{0}=R_{0}(\theta)>1\) such that, for all z satisfying \(\arg z=\theta\) and \(z=r>R_{0}\), we have
$$ \biggl\vert \frac{f^{(j)}(z)}{f^{(i)}(z)}\biggr\vert \leq B\bigl[T(\alpha r,f)\log T( \alpha r,f)\bigr]^{ji}\quad (0\leq i< j). $$(2.2)
Remark 2.1
We use \(E_{2}\subset(0,\infty)\) to denote a set of r of finite logarithmic measure throughout this paper, not necessarily the same at each occurrence.
Lemma 2.3
(see [18])
Let \(f(z)\) be a transcendental entire function, and let \(z_{r}=re^{i\theta_{r}}\) be a point satisfying \(f(z_{r})=M(r,f)\), then there exists a constant \(\delta_{r}>0\) (depending on r) such that, for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\), we have
Lemma 2.4
(see [19])
Let \(f(z)\) be an entire function satisfying \(0<\rho(f)=\rho<\infty\), \(0<\tau(f)=\tau<\infty\). Then for any \(\beta<\tau\), there exists a set \(E_{3}\subset[1,+\infty)\) that has an infinite logarithmic measure such that, for all \(r\in E_{3}\), we have
Lemma 2.5
Let \(f(z)\) be a transcendental entire function satisfying \(0<\rho(f)=\rho<\infty\), \(\tau(f)=\tau>0\), and \(T(r,f)\sim\log M(r,f)\) as \(r\rightarrow\infty\) outside a set of r of finite logarithmic measure. Then for any \(\beta<\tau\), there exists a set \(E_{3}\subset(0,\infty)\) having infinite logarithmic measure and a set \(H_{2}\subset[0,2\pi)\) with linear measure zero such that, for all z satisfying \(z=r\in E_{3}\) and \(\arg z=\theta\in[0,2\pi)\backslash H_{2}\), we have
Proof
Since \(m(r,f)\sim\log M(r,f)\) as \(r\rightarrow\infty\) (\(r\notin E_{2}\)), by the definition of \(m(r,f)\), we see that there exists a set \(H_{2}\subset[0,2\pi)\) with linear measure zero such that for all z satisfying \(\arg z=\theta\in[0,2\pi)\backslash H_{2}\) and for any \(\varepsilon>0\), we have
Otherwise, we find that there exists a set \(H\subset[0,2\pi)\) with positive linear measure, i.e., \(mH>0\) such that, for all z satisfying \(\arg z=\theta\in H\) and for any \(\varepsilon>0\), one has
Then, for all \(r\notin E_{2}\), we have
Since \(\varepsilon>0\), \(mH>0\), (2.7) is a contradiction with \(m(r,f)\sim \log M(r,f)\).
For any \(\beta<\tau\), we choose \(\beta_{1}\) satisfying \(\beta<\beta_{1}<\tau \), by Lemma 2.4, there exists a set \(E_{3}\subset(0,\infty)\) having infinite logarithmic measure such that, for all \(z=r\in E_{3}\), we have
By (2.6) and (2.8), for any given \(0<\varepsilon<1\frac{\beta}{\beta_{1}}\) and, for all z satisfying \(z=r\in E_{3}\backslash E_{2}\) and \(\arg z=\theta\in [0,2\pi)\backslash H_{2}\), we have
Therefore we complete the proof of Lemma 2.5. □
Remark 2.2
The following lemma is a special case of Lemma 29 in [12].
Lemma 2.6
(see [18])
Let \(f(z)\) be a transcendental entire function satisfying \(0<\rho(f)=\rho<\infty\) and \(T(r,f)\sim\log M(r,f)\) as \(r\rightarrow\infty\) outside a set of r of finite logarithmic measure. Then for any given \(\varepsilon>0\), there exists a set \(E_{4}\subset (0,\infty)\) with positive upper logarithmic density and a set \(H_{2}\subset[0,2\pi)\) with linear measure zero such that, for all z satisfying \(r\in E_{4}\) and \(\arg z=\theta\in[0,2\pi)\backslash H_{2}\), we have
Lemma 2.7
(see [20])
Let \(f(z)\) be a meromorphic function of finite order ρ, for any given \(\xi>0\) and l (\(0< l<\frac{1}{2}\)), there exist a constant \(K(\rho,\xi)\) and a set \(E_{\xi}\subset(0,\infty)\) of lower logarithmic density greater than \(1\xi\) such that, for all \(r\in E_{\xi}\) and for J of length l, we have
Lemma 2.8
(see [21])
Let \(A_{j}\) (\(j=0,\ldots,k1\)), \(F\not\equiv0\) be entire functions. If f is a solution of (1.2) satisfying \(\max\{\rho_{2}(F),\rho_{2}(A_{j}):j=0,\ldots,k1\}<\rho_{2}(f)\), then
Lemma 2.9
(see [22])
Suppose that \(P(z)=a_{n}z^{n}+\cdots+a_{0}\) is a polynomial with degree \(n\geq1\), \(a_{n}\in\mathbb{C}\), and that \(A(z)\) (≢0) is an entire function with \(\rho(A)< n\). Set \(g(z)=A(z)e^{P(z)}\), \(z=re^{i\theta}\), \(\delta(P,\theta)=\delta (a_{n}z^{n},\theta)=\operatorname{Re}\{a_{n}e^{in\theta}\}\). Then for any given \(\varepsilon>0\), there exists a set \(H_{3}\subset[0,2\pi)\) of linear measure zero such that for any \(\theta\in[0,2\pi)\backslash H_{3}\), there is a constant \(R(\theta)>0\) such that for \(z=r>R(\theta)\), we have:

(i)
If \(\delta(P,\theta)>0\), then
$$ \exp\bigl\{ (1\varepsilon)\delta(P,\theta)r^{n}\bigr\} < \bigl\vert g \bigl(re^{i\theta}\bigr)\bigr\vert < \exp\bigl\{ (1+\varepsilon)\delta(P, \theta)r^{n}\bigr\} . $$(2.12) 
(ii)
If \(\delta(P,\theta)<0\), then
$$ \exp\bigl\{ (1+\varepsilon)\delta(P,\theta)r^{n}\bigr\} < \bigl\vert g \bigl(re^{i\theta}\bigr)\bigr\vert < \exp\bigl\{ (1\varepsilon)\delta(P, \theta)r^{n}\bigr\} . $$(2.13)
Proofs of Theorems 1.11.3
Proof of Theorem 1.1
(i) By Lemma 2.1, we know that every solution f of (1.2) satisfies \(\rho_{2}(f)\leq\rho(A_{d})\). In the following, we show that every transcendental solution \(f(z)\) of (1.2) satisfies \(\rho_{2}(f)\geq\rho(A_{d})\). Suppose that \(f(z)\) is a transcendental solution of (1.2). By (1.2), we have
For each sufficiently large circle \(z=r\), we take a point \(z_{r}=re^{i\theta_{r}}\) satisfying \(f(z_{r})=M(r, f)>1\). By Lemma 2.3, there exist a constant \(\delta_{r}>0\) and a set \(E_{2}\) such that, for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\), we have
By Lemma 2.2, there exist a set \(H_{1}\subset[0,2\pi)\) having linear measure zero and a constant \(B>0\) such that, for all z satisfying \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\backslash H_{1}\) and for sufficiently large r, we have
We choose \(\alpha_{1}\), \(\alpha_{2}\) satisfying \(\max\{\tau(A_{j}):\rho (A_{j})=\rho(A_{d}),\tau(F)\}<\alpha_{1}<\alpha_{2}<\tau(A_{d})\), since \(f(z)f(z_{r})<\varepsilon\) and \(f(z_{r})\rightarrow\infty\) as \(r\rightarrow\infty\), for all sufficiently large \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\), we have
Since \(T(r,A_{d})\sim\log M(r,A_{d})\) as \(r\rightarrow\infty\) (\(r\notin E_{2}\)), by Lemma 2.5, for any \(\alpha_{2}<\tau(A_{d})\), there exist a set \(E_{3}\subset(0,\infty)\) having infinite logarithmic measure and a set \(H_{2}\subset[0,2\pi)\) with linear measure zero such that for all z satisfying \(z=r\in E_{3}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\backslash H_{2}\), we have
Substituting (3.2)(3.5) into (3.1), for all z satisfying \(z=r\in E_{3}\backslash E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\backslash (H_{1}\cup H_{2})\), we have
From (3.6), we have \(\rho_{2}(f)\geq\rho(A_{d})\). Therefore every transcendental solution \(f(z)\) of (1.2) satisfies \(\rho_{2}(f)=\rho(A_{d})\). If \(f(z)\) is a polynomial solution of (1.2) with \(\deg f \geq d\), then by a simple estimation on both sides of (1.2), we have \(\rho(f^{(k)}+A_{k1}f^{(k1)}+\cdots+A_{0}f)=\rho(F)<\rho(A_{d})\), this is a contradiction, therefore each polynomial solution f of (1.2) must satisfy \(\deg f< d\).
(ii) If \(F\not\equiv0\), by Lemma 2.8, we have that every transcendental solution \(f(z)\) of (1.2) satisfies \(\overline{\lambda}_{2}(f)=\lambda_{2}(f)=\rho_{2}(f)=\rho(A_{d})\).
(iii) If \(d=1\), it is easy to see that (1.2) cannot have polynomial solutions, and by (i) and (ii), we see that every nonconstant solution \(f(z)\) of (1.2) satisfies \(\rho_{2}(f)=\rho(A_{1})\) and \(\overline{\lambda}_{2}(f)=\lambda _{2}(f)=\rho_{2}(f)=\rho(A_{1})\) if \(F\not\equiv0\). □
Proof of Theorem 1.2
Suppose that \(A_{d}(z)\) has \(a\in\mathbb{C}\) as a finite deficient value and satisfying \(\delta(a,A_{d})=2\beta>0\). Then by the definition of deficiency, for sufficiently large r, we have \(m(r,\frac{1}{A_{d}a})>\beta T(r,A_{d})\). Hence, for sufficiently large r, there exists a point \(z_{r}=re^{i\theta_{r}}\) satisfying \(z_{r}=r\) and
Without loss of generality, we assume that \(a=0\). Set \(z_{r}=re^{i\theta_{r}}\), by Lemma 2.7, for any given ξ (\(0<\xi<1\)) and for any given l (\(0< l<\frac{1}{2}\)), there exists a set \(E_{\xi}\subset(0,\infty)\) of lower logarithmic density greater than \(1\xi\) such that, for all z satisfying \(z=r\in E_{\xi}\) and \(\arg z=\theta\in[\theta_{r},\theta_{r}+l]\), we have
We choose l sufficiently small such that \(K(\rho(A_{d}),\xi)(l\log\frac{1}{l})<\beta\), then, for all \(\theta\in[\theta_{r},\theta_{r}+l]\), we have
In general, if \(a\neq0\), then \(A_{d}(z)a\) has zero as a deficient value, and using the reasoning above to \(A_{d}(z)a\), we have
holds, for all \(z=r\in E_{\xi}\) and \(\arg z=\theta\in [\theta_{r},\theta_{r}+l]\). From (3.10), we have
holds, for all z satisfying \(z=r\in E_{\xi}\) and \(\arg z=\theta\in[\theta_{r},\theta_{r}+l]\). Let \(f\not\equiv0\) be a solution of (1.2). By (1.2), we have
By Lemma 2.2, there exists a set \(H_{1}\subset[0,2\pi)\) having linear measure zero and a constant \(B>0\) such that, for all z satisfying \(\arg z=\theta\in[\theta_{r},\theta_{r}+l]\backslash H_{1}\) and for all sufficiently large r, we have
Since \(T(r,A_{0})\sim\log M(r,A_{0})\) as \(r\rightarrow\infty\) (\(r\notin E_{2}\)), by Lemma 2.6, for any given \(\varepsilon>0\), there exists a set \(E_{4}\subset(0,\infty)\) with positive upper logarithmic density and a set \(H_{2}\subset[0,2\pi)\) with linear measure zero such that, for all z satisfying \(z=r\in E_{4}\) and \(\arg z=\theta\in[\theta_{r},\theta_{r}+l]\backslash H_{2}\), we have
Set \(\max\{\rho(A_{j}),j\neq0,d\}=b<\rho(A_{0})\), then for any given ε (\(0<2\varepsilon<\rho(A_{0})b\)) and, for all sufficiently large \(z=r\), we have
Substituting (3.11), (3.13)(3.15) into (3.12), for all \(z=r\in (E_{\xi}\cap E_{4})\backslash E_{2}\) and \(\arg z=\theta\in[\theta_{r},\theta_{r}+l]\backslash(H_{1}\cup H_{2})\), we have
where \((E_{\xi}\cap E_{4})\backslash E_{2}\) is a set having positive upper logarithmic density. From (3.16), we have \(\rho _{2}(f)\geq \rho(A_{0})\). On the other hand, by Lemma 2.1, we see that \(\rho_{2}(f)\leq\rho(A_{d})\) holds, for all solutions of (1.2). Therefore, each solution \(f\not\equiv0\) of (1.2) satisfies \(\rho(A_{0})\leq\rho_{2}(f)\leq\rho(A_{d})\). □
Proof of Theorem 1.3
(1) By Lemma 2.1, it is easy to see that every solution \(f\not\equiv0\) of (1.7) satisfies \(\rho _{2}(f)\leq n\) and that (1.7) has no polynomial solutions by the assumption. In the following, we need to show that every transcendental solution f of (1.7) satisfies \(\rho_{2}(f)\geq n\). We divide the proof into two parts: (i) \(\arg a_{n}\neq\arg b_{n}\) or \(a_{n}=cb_{n}\) (\(0< c<1\)), (ii) \(a_{n}=cb_{n}\) (\(c>1\)).
(i) \(\arg a_{n}\neq\arg b_{n}\) or \(a_{n}=cb_{n}\) (\(0< c<1\)). By Theorem D(i), every solution \(f\not\equiv0\) of (1.7) satisfies \(\rho_{2}(f)\geq n\).
(ii) \(a_{n}=cb_{n}\) (\(c>1\)). We have \(\delta(a_{n}z^{n},\theta)=c\delta(b_{n}z^{n},\theta)\) (\(c>1\)). For each sufficiently large circle \(z=r\), if \(z_{r}=re^{i\theta_{r}}\) is a point satisfying \(f(z_{r})=M(r,f)\), then we affirm that for any given (sufficiently small in general) \(\delta_{r}>0\), we have \([\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\}\neq\emptyset\) and \(m([\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\). On the contrary, if \(z_{r}=re^{i\theta_{r}}\) is a point satisfying \(f(z_{r})=M(r,f)\), and there exists a \(\delta_{1}>0\) (depending on r, in the same way as the following \(\delta_{j}\), \(j=2,3,4,5\)) such that \([\theta_{r}\delta_{1},\theta_{r}+\delta_{1}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\}=\emptyset\), i.e., \([\theta_{r}\delta_{1},\theta_{r}+\delta_{1}]\subset \{\theta:\delta(a_{n}z^{n},\theta)\leq0\}\), we will get a contradiction. In fact, we can choose a \(\delta_{2}>0\) (\(\delta_{2}<\delta _{1}\)) such that \([\theta_{r}\delta_{2},\theta_{r}+\delta_{2}]\subset \{\theta:\delta(a_{n}z^{n},\theta)<0\}\), by (1.7), we have
On each sufficiently large circle \(z=r\), we take a point \(z_{r}=re^{i\theta_{r}}\) such that \(f(z_{r})=M(r,f)\) and \([\theta_{r}\delta_{2},\theta_{r}+\delta_{2}]\subset \{\theta:\delta(a_{n}z^{n},\theta)<0\}\). By Lemma 2.3, there exists a constant \(\delta_{3}=\min\{\delta,\delta_{2}\}>0\) such that for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{3},\theta_{r}+\delta_{3}]\), we have
Since \(\max\{\rho(A_{0}),\rho(A_{1})\}< n\), by Lemma 2.9, for any given \(\varepsilon>0\), there exists a set \(H_{3}\subset[0,2\pi)\) of linear measure zero such that, for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{3},\theta_{r}+\delta_{3}]\backslash H_{3}\), we have
Substituting (3.18)(3.20) into (3.17), we get \(1\leq0\), this is a contradiction. Therefore, for each sufficiently large circle \(z=r\notin E_{2}\), if \(z_{r}=re^{i\theta_{r}}\) is a point satisfying \(f(z_{r})=M(r,f)\), then for any given (sufficiently small) \(\delta_{r}>0\), we have \([\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\}\neq\emptyset\) and \(m([\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\). Then by (1.7), we have
On each sufficiently large circle \(z=r\notin E_{2}\), we choose a point \(z_{r}=re^{i\theta_{r}}\) such that \(f(z_{r})=M(r,f)\) and \(m([\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\cap \{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\). By Lemma 2.2 and Lemma 2.3, for all z satisfying \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\backslash H_{1}\), we have
Since \(\delta(a_{n}z^{n},\theta)=c\delta(b_{n}z^{n},\theta)>0\), by Lemma 2.9, for any given ε (\(0<\varepsilon<\frac{c1}{c+1}\)), there exists a set \(H_{3}\subset[0,2\pi)\) of linear measure zero such that, for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{r},\theta_{r}+\delta_{r}]\backslash (H_{1}\cup H_{3})\), we have
Substituting (3.22) and (3.23) into (3.21), for any given ε (\(0<\varepsilon<\frac{c1}{c+1}\)) and for sufficiently large \(r\notin E_{2}\), we have
By (3.24), we have \(\rho_{2}(f)\geq n\).
Combining (i) and (ii), we have every solution \(f\not\equiv0\) of (1.7) satisfies \(\rho_{2}(f)= n\).
(2) By Lemma 2.1, it is easy to see that every solution f of (1.7) satisfies \(\rho_{2}(f)\leq n\). It is easy to know that (1.7) has no polynomial solutions by the assumption. In the following, we need to show that every transcendental solution f of (1.7) satisfies \(\rho_{2}(f)\geq n\). Since \(a_{n}=cb_{n}\) (\(c<0\)), then we have \(\{\theta:\delta(a_{n}z^{n},\theta)>0\}\cap \{\theta:\delta(b_{n}z^{n},\theta)>0\}=\emptyset\) and \(\{\theta:\delta(a_{n}z^{n},\theta)>0\}\cup \{\theta:\delta(b_{n}z^{n},\theta)>0\}\cup H_{3}=[0,2\pi)\), where \(H_{3}\subset[0,2\pi)\) is a set of linear measure zero. For each sufficiently large circle \(z=r\), we have if \(z_{r}=re^{i\theta_{r}}\) is a point satisfying \(f(z_{r})=M(r,f)\), for any given \(\delta_{4}>0\), set \(I=[\theta_{r}\delta_{4},\theta_{r}+\delta_{4}]\), then we have either \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\) or \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)<0\})>0\). We divide the proof into two cases: (i) \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\), (ii) \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)<0\})>0\).
(i) \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)>0\})>0\). Suppose that \(f(z)\) is a transcendental solution of (1.7), by (1.7), we have
On each sufficiently large circle \(z=r\), we choose a point \(z_{r}=re^{i\theta_{r}}\) satisfying \(f(z_{r})=M(r,f)\). By Lemma 2.2 and Lemma 2.3, there exists a constant \(\delta_{5}=\min\{\delta_{r},\delta_{4}\}>0\) such that, for all z satisfying \(z=r\notin E_{2}\) and \(\arg z=\theta\in[\theta_{r}\delta_{5},\theta_{r}+\delta_{5}]\backslash H_{1}\), we have
Since \(\max\{\rho(A_{0}),\rho(A_{1})\}< n\), by Lemma 2.9, for any given \(\varepsilon>0\), there exists a set \(H_{3}\subset[0,2\pi)\) of linear measure zero such that, for all z satisfying \(\arg z=\theta\in[\theta_{r}\delta_{5},\theta_{r}+\delta_{5}]\backslash H_{3}\), we have
Substituting (3.26)(3.28) into (3.25), for any given ε (>0) and for sufficiently large \(r\notin E_{2}\), we have
By (3.29), we have \(\rho_{2}(f)\geq n\).
(ii) \(m(I\cap\{\theta:\delta(a_{n}z^{n},\theta)<0\})>0\). Replacing \(A_{1}e^{P(z)}\) with \(A_{0}e^{Q(z)}\) on the left side of (3.25) and by the same reasoning in case (i), we can obtain \(\rho_{2}(f)\geq n\) for every transcendental solution of (1.7).
Combining (i) and (ii), every solution f of (1.7) satisfies \(\rho_{2}(f)= n\). Since \(F(z)\not\equiv0\), by Lemma 2.8, every solution f of (1.7) satisfies \(\overline{\lambda}_{2}(f)=\lambda_{2}(f)=\rho_{2}(f)=n\). □
References
 1.
Hayman, WK: Meromorphic Functions. Clarendon, Oxford (1964)
 2.
Laine, I: Nevanlinna Theory and Complex Differential Equations. de Gruyter, Berlin (1993)
 3.
Yi, HX, Yang, CC: The Uniqueness Theory of Meromorphic Functions. Science Press, Beijing (1995) (in Chinese)
 4.
Chen, ZX, Yang, CC: Some further results on the zeros and growth of entire solutions of second order differential equations. Kodai Math. J. 22, 273285 (1999)
 5.
Gundersen, G: Finite order solutions of second order linear differential equations. Trans. Am. Math. Soc. 305, 415429 (1988)
 6.
Hellerstein, S, Miles, J, Rossi, J: On the growth of solutions of \(f''+gf'+hf=0\). Trans. Am. Math. Soc. 324, 693706 (1991)
 7.
Kwon, KH: On the growth of entire functions satisfying second order linear differential equations. Bull. Korean Math. Soc. 33(2), 487496 (1996)
 8.
Kwon, KH: Nonexistence of finite order solutions of certain second order linear differential equations. Kodai Math. J. 19, 378387 (1996)
 9.
Kwon, KH, Kim, JH: Maximum modulus, characteristic, deficiency and growth of solution of second order linear differential equations. Kodai Math. J. 24, 344351 (2001)
 10.
Laine, I, Wu, PC: Growth of solutions of second order linear differential equations. Proc. Am. Math. Soc. 128, 26932703 (2000)
 11.
Wu, PC, Zhu, J: On the growth of solutions to the complex differential equation \(f''+A(z)f'+B(z)f=0\). Sci. China Ser. A 54(5), 939947 (2011)
 12.
Tu, J, Deng, GT: Growth of solutions of a class of higher order linear differential equation. Complex Var. Elliptic Equ. 53(7), 623631 (2008)
 13.
Chen, ZX: The growth of solutions of \(f''+ e^{z}f'+Q(z)f=0\) where the order \((Q)=1\). Sci. China Ser. A 45(3), 290300 (2002)
 14.
Wang, J, Laine, I: Growth of solutions of second order linear differential equations. J. Math. Anal. Appl. 342, 3951 (2008)
 15.
Belaïdi, B: Growth and oscillation related to a second order linear differential equation. Math. Commun. 18(1), 171184 (2013)
 16.
Kinnunen, L: Linear differential equations with solutions of finite iterated order. Southeast Asian Bull. Math. 22(4), 385405 (1998)
 17.
Gundersen, G: Estimates for the logarithmic derivate of a meromorphic function, plus similar estimates. J. Lond. Math. Soc. 37, 88104 (1988)
 18.
Tu, J, Xu, HY, Liu, HM, Liu, Y: Complex oscillation of higher order linear differential equations with coefficients being lacunary series of finite iterated order. Abstr. Appl. Anal. 2013, Article ID 634739 (2013)
 19.
Tu, J, Yi, CF: On the growth of solutions of a class of linear differential equations with coefficients having the same order. J. Math. Anal. Appl. 340(1), 487497 (2008)
 20.
Fuchs, W: Proof of a conjecture of G. Pólya concerning gap series. Ill. J. Math. 7, 661667 (1963)
 21.
Cao, TB, Chen, ZX, Zheng, XM, Tu, J: On the iterated order of meromorphic solutions of higher order linear differential equations. Ann. Differ. Equ. 21(2), 111122 (2005)
 22.
Markushevich, AI: Theory of Functions of a Complex Variable, vol. II. Prentice Hall, Englewood Cliffs (1965)
Acknowledgements
This project is supported by the National Natural Science Foundation of China (11301232, 11301233, 11261024), the Natural Science Foundation of Jiangxi Province in China (20132BAB211002, 20151BAB201004) and the Foundation of Education Bureau of Jiangxi Province in China (GJJ14271, GJJ14272).
Author information
Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
All authors contributed equally to the writing of this paper. 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 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.
About this article
Cite this article
Huang, W.P., Zhou, J.L., Tu, J. et al. On the hyperorder of solutions of two class of complex linear differential equations. Adv Differ Equ 2015, 234 (2015). https://doi.org/10.1186/s136620150571y
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/s136620150571y
MSC
 30D35
 34M10
Keywords
 linear differential equation
 entire function
 hyperorder
 maximum modulus