本文应用Nevanlinna理论及复域微分方程理论[1-2]中的标准记号, 用
近年来, 杨重骏, Laine, 温志涛等人对一些特定形式的微分差分方程的解的情况进行了深入研究, 得出了许多有意义的结论.
杨[3]证明了当
Yang和Laine[4]在此基础上考虑了
随后温志涛[5]对方程做了进一步改进, 研究了方程
李海绸, 高凌云等人[6]从多个方程构成的方程组出发, 研究了方程组
$ \left\{ \begin{align} & \sum\limits_{j=1}^{n}{{{\alpha }_{j}}}(z)f_{1}^{({{\lambda }_{j1}})}(z+{{c}_{j}})={{R}_{2}}(z,{{f}_{2}}(z)), \\ & \sum\limits_{j=1}^{n}{{{\beta }_{j}}}(z)f_{2}^{({{\lambda }_{j2}})}(z+{{c}_{j}})={{R}_{1}}(r,{{f}_{1}}(z)) \\ \end{align} \right. $ |
的情况, 其中
本文将考虑某些类微分差分方程构成的方程组.首先给出非线性差分方程
定理0.1 若
$ \left\{ \begin{align} & f_{1}^{n}-{{p}_{1}}(z){{f}_{2}}(z+c)={{h}_{1}}(z), \\ & f_{2}^{n}-{{p}_{2}}(z){{f}_{1}}(z+c)={{h}_{2}}(z) \\ \end{align} \right. $ | (1) |
的有限级超越整函数解, 那么
受到杨重骏, Laine和温志涛等人结果的启发, 我们对一些特定的微分差分方程组进行了研究, 得出了下面的定理0.2和定理0.3.
定理0.2 已知
$ \left\{ \begin{align} & {{L}_{1}}({{f}_{1}})-p(z)f_{2}^{{{n}_{1}}}(z)={{h}_{1}}(z), \\ & {{L}_{2}}({{f}_{2}})-p(z)f_{1}^{{{n}_{2}}}(z)={{h}_{2}}(z), \\ \end{align} \right. $ | (2) |
则有
定理0.3 已知
$ \left\{ \begin{align} & {{f}_{1}}{{(z)}^{n}}+{{q}_{1}}(z){{e}^{{{Q}_{1}}(z)}}{{f}_{2}}(z+c)={{p}_{1}}(z), \\ & {{f}_{2}}{{(z)}^{n}}+{{q}_{2}}(z){{e}^{{{Q}_{2}}(z)}}{{f}_{1}}(z+c)={{p}_{2}}(z), \\ \end{align} \right. $ | (3) |
的有限级整函数解, 则有
李海绸、高凌云等人考虑了两个方程构成的方程组的情况[6].若是
定理0.4 已知
$ \left\{ \begin{align} & \sum\limits_{j=1}^{n}{{{\alpha }_{1j}}}(z)f_{1}^{({{\lambda }_{1j}})}(z+{{c}_{j}})={{R}_{n}}(z,{{f}_{n}}(z)) \\ & \sum\limits_{j=1}^{n}{{{\alpha }_{2j}}}(z)f_{2}^{({{\lambda }_{2j}})}(z+{{c}_{j}})={{R}_{1}}(z,{{f}_{1}}(z)) \\ & \cdots \cdots \\ & \sum\limits_{j=1}^{n}{{{\alpha }_{nj}}}(z)f_{n}^{({{\lambda }_{nj}})}(z+{{c}_{j}})={{R}_{n-1}}(z,{{f}_{n-1}}(z)) \\ \end{align} \right. $ | (4) |
的超越亚纯解.其中
$ \begin{align*} d_i=\text{deg} R_i(z, f_i(z))=\max \{p_i, q_i\}, \lambda_i=\sum^n_{j=1}(\lambda_{ij}+1), i=1, 2, \cdots, n. \end{align*} $ |
若
为了能够证明定理0.1-0.4,我们需要以下引理.
引理1.1[8] 若
$ \begin{align*} m\Big(r, \frac{f(z+c)}{f(z)}\Big)+m\Big(r, \frac{f(z)}{f(z+c)}\Big)=O(r^{\sigma-1+\varepsilon}) +O(\log r). \end{align*} $ |
引理1.2[6] 若
$ \begin{align*} T(r, f^{(k)})\leq(k+1)T(r, f)+S(r, f). \end{align*} $ |
引理1.3[7] 已知
$ \begin{align*} R(z, f(z))=\frac{P(z, f(z))}{Q(z, f(z))}=\frac{ \sum^p_{i=0}a_i(z)f^i}{\sum^q_{j=0}b_j(z)f^j}, \end{align*} $ |
其中
$ \begin{align*} &T(r, a_i)=S(r, f), i=0, 1, 2, \cdots, p, \\ &T(r, b_j)=S(r, f), j=0, 1, 2, \cdots, q. \end{align*} $ |
那么
$ \begin{align*} T(r, R(z, f))=\max\{p, q\}T(r, f)+S(r, f). \end{align*} $ |
引理1.4[8] 若
$ \begin{align*} T(r, f(z+c))=T(r, f(z))+O(r^{\rho-1+\varepsilon})+O(\log r). \end{align*} $ |
假设
情形1:
$ \left\{ \begin{align} & f_{1}^{n}(z)-{{p}_{1}}(z){{g}_{2}}(z+c)={{h}_{1}}(z), \\ & g_{2}^{n}(z)-{{p}_{2}}(z){{f}_{1}}(z+c)={{h}_{2}}(z). \\ \end{align} \right. $ | (5) |
比较方程组(1)、(5), 得
情形2:
$ \left\{ \begin{align} &g_1^n-p_1(z)g_2(z+c)=h_1(z), \\ &g_2^n-p_2(z)g_1(z+c)=h_2(z). \end{align} \right. $ | (6) |
由方程组(1)、(6)易知
$ \left\{ \begin{align} &f_1^n-g_1^n=p_1(z)(f_2(z+c)-g_2(z+c)), \\ &f_2^n-g_2^n=p_2(z)(f_1(z+c)-g_1(z+c)). \end{align} \right. $ |
两边分别同除
$ \left\{ \begin{align} &\frac{f_1^n-g_2^n}{f_1-g_1}=p_1(z)\frac{f_2(z+c)-g_2(z+c)}{f_1-g_1}, \\ &\frac{f_2^n-g_2^n}{f_2-g_2}=p_2(z)\frac{f_1(z+c)-g_1(z+c)}{f_2-g_2}. \end{align} \right. $ | (7) |
令
$ \begin{align*} m(r, F)&=m\Big(r, \frac{(f_2^n-g_2^n)(f_1^n-g_1^n)}{(f_1-g_1)(f_2-g_2)}\Big)\\ &=m\Big(r, p_1(z)p_2(z)\frac{(f_2(z+c)-g_2(z+c))(f_1(z+c)-g_1(z+c))}{(f_1-g_1)(f_2-g_2)}\Big)\\ &\leq m\Big(r, \frac{f_1(z+c)-g_1(z+c)}{f_1-g_1}\Big)+m\Big(r, \frac{f_2(z+c)-g_2(z+c)}{f_2-g_2}\Big) \\ & +m(r, p_1)+m(r, p_2)+O(\log r) \\ &\leq O(r^{\sigma(f_1-g_1)-1+\varepsilon})+O(r^{\sigma(f_2-g_2)-1+\varepsilon})+O(\log r). \end{align*} $ |
令
$ \begin{align*} N\Big(r, \frac{1}{f_i-\eta_jg_i}\Big)=S_\sigma(r). \end{align*} $ |
进一步有
$ \begin{equation} N\Big(r, \frac{1}{\frac{f_i}{g_i}-\eta_j}\Big)=S_\sigma(r). \end{equation} $ | (8) |
又对
$ \begin{equation} T(r, f_i-g_i)=T\Big(r, g_i\Big(\frac{f_i}{g_i}-1\Big)\Big)\leq T(r, g_i)+T\Big(r, \frac{f_i}{g_i}-1\Big). \end{equation} $ | (9) |
由第二基本定理以及(8)式可得
$ \begin{align*} T(r, f_i-g_i)\leq T(r, g_i)+S_\sigma(r). \end{align*} $ |
因为
$ \begin{align*} (n-1)T(r, g_i)\leq T\Big(r, \frac{f_i^n-g_i^n}{f_i-g_i}\Big)+T\left(r, \prod^{n-1}_{j=1} \left(\frac{1}{\frac{f_i}{g_i}-\eta_j}\right)\right) \end{align*} $ |
成立.因为
$ \begin{align*} (n-1)T(r, g_i)\leq S_\sigma(r)+T\Big(r, \prod^{n-1}_{j=1} \Big(\frac{f_i}{g_i}-\eta_j\Big)\Big) +O(1). \end{align*} $ |
再次利用第二基本定理以及(8)式, 有
$ \begin{align*} (n-1)T(r, g_i)\leq S_\sigma(r). \end{align*} $ |
代入(9)式, 得
$ \begin{align*} T(r, f_1-g_1)+T(r, f_2-g_2)\leq O(r^{\sigma(f_1-g_1)-1+\varepsilon})+ O(r^{\sigma(f_2-g_2)-1+\varepsilon})+O(\log r)=S_\sigma(r). \end{align*} $ |
易知存在某个
那么, 当
$ \left\{ \begin{align} &\eta_i^nf_1^n-p_1g_2(z+c)=h_1(z), \\ &g_2^n-p_2\eta_if_1(z+c)=h_2(z). \end{align} \right. $ | (10) |
因为
$ \begin{equation} f_2(z+c)=g_2(z+c). \end{equation} $ | (11) |
由方程组(1)和(6), 有
$ \left\{ \begin{align} &(f_2^n(z)-h_2(z))^n=(p_2(z)f_1(z+c))^n, \\ &(g_2^n(z)-h_2(z))^n=(p_2(z)g_1(z+c))^n. \end{align} \right. $ | (12) |
由前述
$ \begin{align*} (f_2^n(z)-h_2(z))^n=(g_2^n(z)-h_2(z))^n. \end{align*} $ |
则有
$ \begin{align*} f_2^n(z+c)-h_2(z+c)=\varphi_i(g_2^n(z+c)-h_2(z+c)). \end{align*} $ |
结合(11)式得
当
$ \begin{equation} F(z)=q(z)\not\equiv0. \end{equation} $ | (13) |
当
$ \begin{align*} F=(f_1-\eta_1g_1)(f_2-\eta_1g_2)(f_1-\eta_2g_1)(f_2-\eta_2g_2) \cdots (f_1-\eta_{n-1}g_1)(f_2-\eta_{n-1}g_2)=q(z). \end{align*} $ |
由于
对方程组(2)的第一个式子取Nevanlina特征函数并结合引理(1.2), 得
$ \begin{align*} n_1T(r, f_2)&=T(r, L_1(f_1)-h_1(z))\\ &\leq T(r, L_1(f_1))+O(\log r)\\ &\leq \sum^{k_1}_{j=1}T(r, f_1^{(j)})+T(r, f_1)+O(\log r)\\ &\leq \frac{(k_1+1)(k_1+2)}{2}T(r, f_1)+k_1S(r, f_1)+O(\log r). \end{align*} $ |
令
$ \begin{equation} n_1T(r, f_2)\leq d_1T(r, f_1)+k_1S(r, f_1)+O(\log r). \end{equation} $ | (14) |
同理
$ \begin{equation} n_2T(r, f_1)\leq d_2T(r, f_2)+k_2S(r, f_2)+O(\log r). \end{equation} $ | (15) |
则将(14)式乘
$ \begin{align} &d_2n_1T(r, f_2)\leq d_1d_2T(r, f_1)+d_2k_1S(r, f_1)+O(\log r), \end{align} $ | (16) |
$ \begin{align} &n_1n_2T(r, f_1)\leq n_1d_2T(r, f_2)+n_1k_2S(r, f_2)+O(\log r). \end{align} $ | (17) |
由(16)、(17)式得
$ \begin{equation} n_1n_2T(r, f_1)\leq d_1d_2T(r, f_1)+d_2k_1S(r, f_1)+n_1k_2S(r, f_2)+O(\log r). \end{equation} $ | (18) |
同理, 将(14)式乘
$ \begin{equation} n_1n_2T(r, f_2)\leq d_1d_2T(r, f_2)+d_1k_2S(r, f_2)+n_2k_1S(r, f_1)+O(\log r). \end{equation} $ | (19) |
由(18)、(19)式得
$ \begin{align*} (n_1n_2-d_1d_2)(T(r, f_1)+T(r, f_2)) \leq(d_2k_1+n_2k_1)S(r, f_1)+(n_1k_2+d_1k_2)S(r, f_2)+O(\log r). \end{align*} $ |
因为
假设
$ \begin{align*} nm(r, f_1)&=m(r, p_1(z)-q_1(z)f_2(z+c)\text{e}^{Q_1(z)})\\ &\leq m(r, p_1(z))+m(r, q_1(z))+m(r, f_2(z+c))+m(r, \text{e}^{Q_1(z)})+O(1)\\ &\leq m(r, \text{e}^{Q_1(z)})+m\Big(r, \frac{f_2(z+c)}{f_2(z)}\Big)+m(r, f_2(z))+O(\log r)\\ &\leq m(r, \text{e}^{Q_1(z)})+m(r, f_2(z))+O(r^{\sigma(f_2)-1+\varepsilon})+O(\log r), \end{align*} $ |
即
$ \begin{equation} nm(r, f_1)\leq m(r, \text{e}^{Q_1})+m(r, f_2)+O(r^{\sigma(f_2)-1+\varepsilon})+O(\log r). \end{equation} $ | (20) |
同理有
$ \begin{equation} nm(r, f_2)\leq m(r, \text{e}^{Q_2})+m(r, f_1)+O(r^{\sigma(f_1)-1+\varepsilon})+O(\log r). \end{equation} $ | (21) |
由(20)、(21)式得
$ \begin{align*} (n^2-1)m(r, f_2)\leq m(r, \text{e}^{Q_1})+nm(r, \text{e}^{Q_2})+nO(r^{\sigma(f_1)-1+\varepsilon})+O(r^{\sigma(f_2)-1+\varepsilon})+(n+1)O(\log r). \end{align*} $ |
且当
又由方程组(3), 有
$ \begin{align*} m(r, e^{Q_1})&=m\Big(r, \frac{p_1(z)-f_1(z)^{n}}{q_1(z)f_2(z+c)}\Big)\\ &\leq m\Big(r, \frac{p_1(z)}{q_1(z)f_2(z+c)}\Big)+m\Big(r, \frac{f_1(z)^n}{q_1(z)f_2(z+c)}\Big)+O(1)\\ &\leq m\Big(r, \frac{1}{f_2(z+c)}\Big)+m\Big(r, \frac{f_1^n(z)}{f_2(z+c)}\Big)+O(\log r)\\ &\leq2m\Big(r, \frac{f_2(z)}{f_2(z+c)}\Big)+2m\Big(r, \frac{1}{f_2(z)}\Big)+nm(r, f_1) +O(\log r)\\ &\leq n T(r, f_1)+2T(r, f_2)+O(r^{\sigma(f_2)-1+\varepsilon})+O(\log r), \end{align*} $ |
即
$ \begin{equation} m(r, \text{e}^{Q_1})\leq nT(r, f_1)+2T(r, f_2)+O(r^{\sigma(f_2)-1+\varepsilon}) +O(\log r). \end{equation} $ |
同理有
$ \begin{equation} m(r, \text{e}^{Q_2})\leq nT(r, f_2)+2T(r, f_1)+O(r^{\sigma(f_1)-1+\varepsilon})+O(\log r). \end{equation} $ | (23) |
由(22)、(23)式得
$ \begin{align*} m(r, \text{e}^{Q_1})+m(r, \text{e}^{Q_2})\leq(n+2)[T(r, f_1) +T(r, f_2)]+O(r^{\sigma(f_1)-1+\varepsilon}) +O(r^{\sigma(f_2)-1+\varepsilon})+O(\log r). \end{align*} $ |
即deg
已知方程组(4)中系数
$ \begin{align*} d_nT(r, f_n)&=T(r, R_n)+S(r, f_n)\\ &=T\Big(r, \sum^n_{j=1}\alpha_{1j}(z)f_1^{(\lambda_{1j})}(z+c_j)\Big)+S(r, f_n)\\ &\leq\sum^n_{j=1}T(r, f_1^{(\lambda_{1j})}(z+c_j))+S(r, f_n)\\ &\leq\sum^n_{j=1}(\lambda_{1j}+1)T(r, f_1(z+c_j))+S(r, f_n)\\ &=\lambda_1T(r, f_1)+O(r^{\rho(f_1)-1+\varepsilon})+O(\log r)+S(r, f_n). \end{align*} $ |
设
$ \begin{equation} (d_n+o(1))T(r, f_n)\leq \lambda_1T(r, f_1)+O(r^{\rho-1+\varepsilon})+O(\log r). \end{equation} $ | (24) |
同理可得
$ \begin{align} &(d_1+o(1))T(r, f_1)\leq\lambda_2T(r, f_2)+O(r^{\rho-1+\varepsilon}) +O(\log r), \notag\\ &\cdots\cdots\\ &(d_{n-1}+o(1))T(r, f_{n-1})\leq \lambda_nT(r, f_n)+O(r^{\rho-1+\varepsilon})+O(\log r).\notag \end{align} $ | (25) |
联立(24)、(25)式得
$ \begin{align*} (d_{n-1}+o(1))T(r, f_{n-1})\leq &\frac{\lambda_1\lambda_2 \cdots\lambda_n}{(d_1+o(1))\cdots (d_{n-2}+o(1))(d_n+o(1))}T(r, f_{n-1})\notag\\ &+M[O(r^{\rho-1+\varepsilon})+O(\log r)], \end{align*} $ |
其中
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