2. 中国矿业大学 数学学院, 江苏 徐州 221116
2. School of Mathematics, China University of Mining and Technology, Xuzhou 221116, Jiangsu Province, China
近年来, 关于有限或无限维Banach空间中演化方程解的渐近行为研究取得了突破性进展, 尤其在指数稳定性理论方面, 取得了丰富的研究成果[1-14].由于在物理学、工程学、经济学中很难找到准确的数学模型去刻画动力系统, 而其演化过程的解可以形成一个线性斜积半流, 故可借助线性斜积半流来讨论该类发展方程的指数渐近行为.关于线性斜积半流的指数稳定性, MEGAN等利用容许性方法、Datko方法和构造函数空间上的泛函方法对其进行了相关讨论[3-7]; 文献[10-14]对斜演化半流的指数稳定性进行了探讨, 从而推广了线性斜积半流的概念.本文将在上述文献的基础上, 对Banach空间中线性斜积半流的一致指数稳定性给出Datko、Rolewicz型刻画及遍历刻画.
第1节给出后文需要的概念; 第2节讨论线性斜积半流的一致指数稳定性, 并给出了若干一致指数稳定的充分或必要条件, 所得结果推广了稳定性理论中的一些经典结论.
1 预备知识设X是一个Banach空间, (Θ, d)为一个度量空间, 将空间X上的范数及其作用于有界线性算子全体B(X)上的范数记作‖·‖, I为恒等算子.
定义1[5] 如果满足性质:
(ⅰ) σ(θ, 0)=θ, ∀θ∈Θ;
(ⅱ) σ(θ, t+s)=σ(σ(θ, s), t), ∀(θ, s, t)∈Θ×R+2.
则称连续映射σ: Θ×R+→Θ为Θ上的半流.
定义2[5] 如果σ为Θ上的半流且Φ: Θ×R+→B(X)满足以下条件:
(ⅰ) Φ(θ, 0)=I, ∀θ∈Θ;
(ⅱ) Φ(θ, t+s)=Φ(σ(θ, t), s)Φ(θ, t), ∀(θ, s, t)∈Θ×R+2;
(ⅲ)存在M≥1和ω>0使得
$ \left\| {\mathit{\Phi }\left( {\theta ,t} \right)} \right\| \le M{{\rm{e}}^{\omega t}},\forall \left( {\theta ,t} \right) \in \mathit{\Theta } \times {{\bf{R}}_ + }; $ |
(ⅳ)对所有的(θ, x)∈Θ×X, 映射R+∋t↦Φ(θ, t)x∈X连续,
则称π=(Φ, σ)为X×Θ上的线性斜积半流.
例1 设σ为度量空间Θ上的半流, X是一个Banach空间, A: Θ→B(X)为连续映射.若Φ(θ, t)x为抽象Cauchy问题:
$ \left\{ \begin{array}{l} u'\left( t \right) = A\left( {\sigma \left( {\theta ,t} \right)} \right)u\left( t \right),\;\;\;\;t \ge 0,\\ u\left( 0 \right) = x \end{array} \right. $ |
的解, 则π=(Φ, σ)为X×Θ上的线性斜积半流.
定义3 如果存在常数N>0和v>0, 使得对∀(t, θ, x)∈R+×Θ×X, 有
$ \left\| {\mathit{\Phi }\left( {\theta ,t} \right)x} \right\| \le N{{\rm{e}}^{ - vt}}\left\| x \right\|, $ | (1) |
则称线性斜积半流π=(Φ, σ)为一致指数稳定的.
2 主要结论及其证明定理1 线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在h>0和c∈(0, 1), 使得对每个θ∈Θ和每个x∈X存在τ∈(0, h], 满足
$ \left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\| \le c\left\| x \right\|. $ | (2) |
证明 必要性显然.下证充分性.
设任一固定的θ∈Θ, x∈X.由假设知, 存在τ1∈(0, h], 使得式(2)成立.对θ1=σ(θ, τ1)和x1=Φ(θ, τ1)x可选择τ2∈(0, h], 使得
$ \left\| {\mathit{\Phi }\left( {{\theta _1},{\tau _2}} \right){x_1}} \right\| \le c\left\| {{x_1}} \right\| \le {c^2}\left\| x \right\|, $ |
由于
$ \begin{array}{l} \mathit{\Phi }\left( {{\theta _1},{\tau _2}} \right){x_1} = \mathit{\Phi }\left( {\sigma \left( {\theta ,{\tau _1}} \right),{\tau _2}} \right)\mathit{\Phi }\left( {\theta ,{\tau _1}} \right)x = \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\mathit{\Phi }\left( {\theta ,{\tau _1} + {\tau _2}} \right)x, \end{array} $ |
不妨记τ0=0, sn=
$ \left\| {\mathit{\Phi }\left( {\theta ,{s_n}} \right)x} \right\| \le {c^n}\left\| x \right\|,\;\;\;\;n \in {\bf{N}}. $ | (3) |
若sn→∞(n→∞), 则对每个t∈[sn, sn+1], 有t≤(n+1)h.从而
$ \begin{array}{l} \left\| {\mathit{\Phi }\left( {\theta ,t} \right)x} \right\| = \left\| {\mathit{\Phi }\left( {\sigma \left( {\theta ,{s_n}} \right),t - {s_n}} \right)\mathit{\Phi }\left( {\theta ,{s_n}} \right)x} \right\| \le \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;M{{\rm{e}}^{\omega h}}{c^n}\left\| x \right\| \le M{{\rm{e}}^{\left( {\omega + v} \right)h}}{{\rm{e}}^{ - vt}}\left\| x \right\|, \end{array} $ |
其中v=
如果sn→s(n→∞), 利用式(3)可得Φ(θ, s)x=0.当t≥s时,
$ \mathit{\Phi }\left( {\theta ,t} \right)x = \mathit{\Phi }\left( {\sigma \left( {\theta ,s} \right),t - s} \right)\mathit{\Phi }\left( {\theta ,s} \right)x = 0, $ |
当0≤t<s时, 类似于sn→∞的情况.
综上可知, 对∀(t, θ, x)∈R+×Θ×X, 式(1)成立.
证毕!
定理2 如果φ, ψ: [0, ∞)→[0, ∞)是2个非减的函数, 满足
$ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\int_0^t {\varphi \left( {\psi \left( \tau \right)\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } < \infty , $ | (4) |
则线性斜积半流π=(Φ, σ)是一致指数稳定的.
证明 结合已知条件, 利用反证法推出式(2)成立.如果式(2)不成立, 则对每个h>0及每个c∈(0, 1)存在x0∈X, ‖x0‖=1及θ0∈Θ, 使得
$ \left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\| > c\left\| {{x_0}} \right\| = c $ |
对所有τ∈(0, h]成立.从而由式(4)可得, 对所有h>0, 有
$ \begin{array}{l} M = \mathop {\sup }\limits_{t > 0} \frac{1}{t}\int_0^t {\varphi \left( {\psi \left( \tau \right)\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|} \right){\rm{d}}\tau } \ge \\ \;\;\;\;\;\;\;\mathop {\sup }\limits_{t \in \left( {0,h} \right]} \frac{1}{t}\int_0^t {\varphi \left( {c\psi \left( \tau \right)} \right){\rm{d}}\tau } \ge \\ \;\;\;\;\;\;\;\frac{1}{h}\int_0^h {\varphi \left( {c\psi \left( \tau \right)} \right){\rm{d}}\tau } , \end{array} $ |
因此, 由L'Hospital法则, 有
$ M \ge \mathop {\lim }\limits_{t \to \infty } \frac{1}{t}\int_0^t {\varphi \left( {c\psi \left( \tau \right)} \right){\rm{d}}\tau } = \mathop {\lim }\limits_{t \to \infty } \varphi \left( {c\psi \left( t \right)} \right) = \infty , $ |
此矛盾说明式(2)成立.从而π=(Φ, σ)是一致指数稳定的.
证毕!
推论1 若φ(t)满足定理2的条件, 则线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在2个常数α>0和β>0使得对所有的(θ, x)∈Θ×X有
$ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\int_0^t {\varphi \left( {{{\rm{e}}^{\alpha \tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } \le \varphi \left( {\beta \left\| x \right\|} \right). $ | (5) |
证明 必要性.设存在常数N>0, v>0, 使得‖Φ(θ, t)‖≤Ne-v t对所有t≥0和θ∈Θ成立, 任意固定的α∈(0, v]及β≥N, 对每个θ∈Θ和每个x∈X, 当t>0时, 有
$ \begin{array}{l} \varphi \left( {\beta \left\| x \right\|} \right) = \frac{1}{t}\int_0^t {\varphi \left( {\beta \left\| x \right\|} \right){\rm{d}}\tau } \ge \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\frac{1}{t}\int_0^t {\varphi \left( {{{\rm{e}}^{\alpha \tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } . \end{array} $ |
由定理2可知充分性显然.
证毕!
推论2 如果φ(t)满足定理2的条件, 且存在3个常数α>0, β>0, γ>0, 使得对所有的(θ, x)∈Θ×X, 有
$ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\varphi \left( {\gamma {{\rm{e}}^{\alpha n}}\left\| {\mathit{\Phi }\left( {\theta ,n} \right)x} \right\|} \right)} \le \varphi \left( {\beta \left\| x \right\|} \right), $ | (6) |
则线性斜积半流π=(Φ, σ)是一致指数稳定的.
证明 利用式(6), 当t>0时有
$ \begin{array}{l} \mathop {\sup }\limits_{t > 0} \frac{1}{t}\int_0^t {\varphi \left( {{{\rm{e}}^{\alpha \tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } \le \\ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\int_n^{n + 1} {\varphi \left( {{{\rm{e}}^{\alpha \tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } } \le \\ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\int_n^{n + 1} {\varphi \left( {{{\rm{e}}^{\alpha \left( {n + 1} \right)}} \times } \right.} } \\ \;\;\;\;\;\;\left. {\left\| {\mathit{\Phi }\left( {\sigma \left( {\theta ,n} \right),\tau - n} \right)\mathit{\Phi }\left( {\theta ,n} \right)x} \right\|} \right){\rm{d}}\tau \le \\ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\int_n^{n + 1} {\varphi \left( {M{{\rm{e}}^{\omega + \alpha \left( {n + 1} \right)}}\left\| {\mathit{\Phi }\left( {\theta ,n} \right)x} \right\|} \right){\rm{d}}\tau } } = \\ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\varphi \left( {M{{\rm{e}}^{\omega + \alpha \left( {n + 1} \right)}}\left\| {\mathit{\Phi }\left( {\theta ,n} \right)x} \right\|} \right)} = \\ \mathop {\sup }\limits_{t > 0} \frac{1}{t}\sum\limits_{n = 0}^{\left[ {t + 1} \right]} {\varphi \left( {\gamma {{\rm{e}}^{\alpha n}}\left\| {\mathit{\Phi }\left( {\theta ,n} \right)x} \right\|} \right)} \le \\ \varphi \left( {\beta \left\| x \right\|} \right), \end{array} $ |
其中γ=Meω+α, M, ω由定义2给出.进而由推论1可知结论成立.
证毕!
定理3 线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在非减函数f(t)>0(t>0)满足f(ts)≤f(t)f(s) (t, s≥0)及K>0, 使得对所有x∈X和θ∈Θ, 有
$ \int_0^\infty {f\left( {\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right){\rm{d}}\tau } \le Kf\left( {\left\| x \right\|} \right). $ | (7) |
证明 必要性.若π=(Φ, σ)是一致指数稳定的, 则存在常数N>0, v>0, 使得对所有x∈X和θ∈Θ, 有
$ \int_0^\infty {\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|{\rm{d}}\tau } \le N\int_0^\infty {{{\rm{e}}^{ - v\tau }}\left\| x \right\|{\rm{d}}\tau } = \frac{N}{v}\left\| x \right\|. $ |
因此, 当f(t)=t, K=N/v时, 式(7)成立.
充分性.如果π=(Φ, σ)非一致指数稳定, 则对每个h>0及每个c∈(0, 1), 存在x0∈X及θ0∈Θ, 使得对所有τ∈(0, h], 有
$ \left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\| > c\left\| {{x_0}} \right\|, $ | (8) |
特别地, 取h=Kf(3)及c=1/3, 由式(8)有
$ \begin{array}{l} f\left( 3 \right)\int_0^\infty {f\left( {\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|} \right){\rm{d}}\tau } \ge \\ \int_0^\infty {f\left( {3\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|} \right){\rm{d}}\tau } \ge \\ \int_0^h {f\left( {\left\| {{x_0}} \right\|} \right){\rm{d}}\tau } = Kf\left( 3 \right)f\left( {\left\| {{x_0}} \right\|} \right). \end{array} $ |
与式(7)矛盾.因此, π=(Φ, σ)是一致指数稳定的.
证毕!
定理4 线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在非减函数f(t)>0(t>0), 满足f(ts)≥f(t)f(s) (t, s≥0)及K>0, 使得对所有x∈X和θ∈Θ, 式(7)成立.
证明 必要性.令f(t)=t即可证得必要性.
充分性.类似定理3.由式(8), 对c∈(0, 1)和h=K/f(c), 有
$ \begin{array}{*{20}{c}} {\int_0^\infty {f\left( {\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|} \right){\rm{d}}\tau } \ge \int_0^h {f\left( {c\left\| {{x_0}} \right\|} \right){\rm{d}}\tau } \ge }\\ {hf\left( c \right)f\left( {\left\| {{x_0}} \right\|} \right) = Kf\left( {\left\| {{x_0}} \right\|} \right).} \end{array} $ |
与式(7)矛盾.因此, π=(Φ, σ)是一致指数稳定的.
证毕!
推论3 线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在p>0及K>0对所有x∈X和θ∈Θ, 有
$ {\left( {\int_0^\infty {{{\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|}^p}{\rm{d}}\tau } } \right)^{\frac{1}{p}}} \le K\left\| x \right\|. $ | (9) |
推论4 若f(t)满足定理3或定理4的条件, 则线性斜积半流π=(Φ, σ)是一致指数稳定的充要条件为存在K>0对所有x∈X和θ∈Θ, 有
$ \sum\limits_{n = 0}^\infty {f\left( {\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|} \right)} \le Kf\left( {\left\| x \right\|} \right). $ | (10) |
定理5 线性斜积半流π=(Φ, σ)是一致指数稳定的当且仅当存在K>0, λ>0对所有t>0及∀(θ, x)∈Θ×X, 有
$ \frac{1}{t}\int_0^t {{{\rm{e}}^{\lambda \tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|{\rm{d}}\tau } \le K\left\| x \right\|. $ | (11) |
证明 必要性.如果π=(Φ, σ)是一致指数稳定的, 则由定义, 存在N>0, v>0, 使得对∀(θ, x)∈Θ×X, 有
$ \begin{array}{l} \frac{1}{t}\int_0^t {{{\rm{e}}^{\frac{v}{2}\tau }}\left\| {\mathit{\Phi }\left( {\theta ,\tau } \right)x} \right\|{\rm{d}}\tau } \le \\ \;\;\;\;\;\frac{N}{t}\int_0^t {{{\rm{e}}^{ - \frac{{v\tau }}{2}}}\left\| x \right\|{\rm{d}}\tau } = \frac{{2N}}{{vt}}\left( {1 - {{\rm{e}}^{ - \frac{{vt}}{2}}}} \right)\left\| x \right\|, \end{array} $ |
令λ=v/2, K=
充分性.类似定理3的证明.如果π=(Φ, σ)不是一致指数稳定的, 设h>0满足eλh>1+3λhK, 其中K, λ由式(11)给出, 由定理1知, 当c=1/3时存在x0∈X及θ0∈Θ对所有τ∈(0, h], 有式(8)成立.从而当t=h时, 有
$ \begin{array}{l} \frac{1}{t}\int_0^t {{{\rm{e}}^{\lambda \tau }}\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|{\rm{d}}\tau } = \\ \;\;\;\;\;\;\frac{1}{h}\int_0^h {{{\rm{e}}^{\lambda \tau }}\left\| {\mathit{\Phi }\left( {{\theta _0},\tau } \right){x_0}} \right\|{\rm{d}}\tau } > \\ \;\;\;\;\;\;\frac{1}{h}\int_0^h {{{\rm{e}}^{\lambda \tau }}\frac{1}{3}\left\| {{x_0}} \right\|{\rm{d}}\tau } = \\ \;\;\;\;\;\;\frac{{{{\rm{e}}^{\lambda h}} - 1}}{{3h\lambda }}\left\| {{x_0}} \right\| > K\left\| {{x_0}} \right\|. \end{array} $ |
与式(11)矛盾, 因此π=(Φ, σ)是一致指数稳定的.
证毕!
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