Weighted piecewise pseudo almost automorphic functions with applications to abstract impulsive ∇-dynamic equations on time scales

In the present paper, by introducing the concept of equipotentially almost automorphic sequence, the concept of weighted piecewise pseudo almost automorphic functions on time scales is proposed. Some first results about their basic properties are obtained and some composition theorems are established. Then we apply these to investigate the existence of weighted piecewise pseudo almost automorphic mild solutions to abstract impulsive ∇-dynamic equations on time scales. In addition, the exponential stability of weighted piecewise pseudo almost automorphic mild solutions is also considered. Finally, the obtained results are applied to the study of a class of ∇-partial differential equations on time scales.

MSC:34N05, 35B15, 43A60, 12H20, 35R12.

Almost automorphic functions, which are more general than almost periodic functions, were introduced by Bochner in relation to some aspects of differential geometry (see [1–3]). For more details as regards this topic we refer to the recent books [4–6], where the authors gave important overviews about the theory of almost automorphic functions and their applications to differential equations. Almost automorphic and pseudo almost automorphic solutions in the context of differential equations had been studied by several authors [7–21]. N’Guérékata [13] and Xiao [15, 21] with their collaborators established the existence and uniqueness theorems of pseudo almost automorphic solutions to some semilinear abstract differential equations. Recently, Blot et al. [22] introduced the concept of weighted pseudo almost automorphic functions, which generalizes the concept of weighted pseudo almost periodicity [23–26], and the author proved some interesting properties of the space of weighted pseudo almost automorphic functions like the completeness and the composition theorem, which have many applications in the context of differential equations. For other contributions to the study of weighted pseudo almost automorphy, we refer the reader to [27–30] and references therein.

On the other hand, the theory of time scales, which has recently received a lot of attention, was introduced by Hilger in his PhD thesis in 1988 [31] in order to unify continuous and discrete analysis. This theory represents a powerful tool for applications to economics, population models, and quantum physics among others. In fact, the progressive field of dynamic equations on time scales contains links to and extends the classical theory of differential and difference equations. For instance, by choosing the time scale to be the set of real numbers, the general result yields a result for differential equations. In a similar way, by choosing the time scale to be the set of integers, the same general result yields a result for difference equations. However, since there are many other time scales than just the set of real numbers or the set of integers, one has a much more general result. For these reasons, based on the concept of almost periodic time scales proposed in [32, 33], the concept of weighted pseudo almost automorphic functions on almost periodic time scales was formally introduced by Wang and Li (2013) in [34]. Moreover, some first results were proven which concern the weighted pseudo almost automorphic mild solution to abstract Δ-dynamic equations on time scales. In addition, by using the results obtained in [32, 33], Lizama and Mesquita [35] presented some new results about basic properties of almost automorphic functions on time scales and proved the existence and uniqueness of an almost automorphic solution to a class of Δ-dynamic equations.

For another thing, many phenomena in nature are characterized by the fact that their states are subject to sudden changes at certain moments and therefore can be described by impulsive system (see [36, 37]). Many evolution processes, optimal control models in economics, stimulated neural networks, population models, artificial intelligence, and robotics are characterized by the fact that at certain moments of time they undergo abrupt changes of state. The existence of almost periodic solutions of abstract impulsive differential equations has been considered by many authors; see [38–41].

However, to the best of our knowledge, the concept of weighted piecewise pseudo almost automorphic functions on time scales has not been introduced in any literature until now, so there was no work on discussing weighted piecewise pseudo almost automorphic problems of impulsive dynamic equations on time scales before. Therefore, in this paper, by introducing the concept of equipotentially almost automorphic sequence, the concept of weighted piecewise pseudo almost automorphic functions on time scales is proposed. The first results about their basic properties are obtained and some composition theorems are established. Then we apply these composition theorems to investigate the existence of weighted piecewise pseudo almost automorphic mild solutions to abstract impulsive ∇-dynamic equations as follows:

where A\in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) is a linear operator in the Banach space \mathbb{X} and f\in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T}\times \mathbb{X},\mathbb{X}), x^{\varrho }=x(\varrho (t)). f, I_i, t_i satisfy suitable conditions that will be established later and \mathbb{T} is an almost periodic time scale. In addition, the notations x(t_i^{+}) and x(t_i^{-}) represent the right-hand and the left-hand side limits of x(\cdot ) at t_i, respectively. In addition, some useful lemmas are obtained and the exponential stability of weighted piecewise pseudo almost automorphic mild solutions is also considered. Finally, we apply these obtained results to study a class of ∇-partial differential equations on time scales.

In the following, we will introduce some basic knowledge of time scales which is very useful to the proof of our relative results.

A time scale \mathbb{T} is a closed subset of ℝ. It follows that the jump operators \sigma ,\varrho :\mathbb{T}\to \mathbb{T} defined by \sigma (t)=inf\{s\in \mathbb{T}:s>t\} and (supplemented by inf\varphi :=sup\mathbb{T} and sup\varphi :=inf\mathbb{T}) are well defined. The point t\in \mathbb{T} is left-dense, left-scattered, right-dense, right-scattered if \varrho (t)=t, , \sigma (t)=t, \sigma (t)>t, respectively. If \mathbb{T} has a right-scattered minimum m, define {\mathbb{T}}_k:=\mathbb{T}\mathrm{\setminus }m; otherwise, set {\mathbb{T}}^k=\mathbb{T}. By the notations {[a,b]}_{\mathbb{T}}, {[a,b)}_{\mathbb{T}} and so on, we will denote time scale intervals

where a,b\in \mathbb{T} with .

The graininess function is defined by \nu :\mathbb{T}\to [0,\mathrm{\infty }): \nu (t):=t-\varrho (t), for all t\in \mathbb{T}.

Definition 2.1 ([42])

The function f:\mathbb{T}\to \mathbb{R} is called ld-continuous provided that it is continuous at each left-dense point and has a right-sided limit at each point, write f\in C_{\mathrm{ld}}(\mathbb{T})=C_{\mathrm{ld}}(\mathbb{T},\mathbb{R}). Let t\in {\mathbb{T}}_k, the Delta derivative of f at t such that

for all s\in U, at fixed t. Let F be a function, it is called the antiderivative of f:\mathbb{T}\to \mathbb{R} provided F^{\mathrm{\nabla }}(t)=f(t) for each t\in {\mathbb{T}}_k. If F^{\mathrm{\nabla }}(t)=f(t), then we define the delta integral by

Definition 2.2 ([42])

A function p:\mathbb{T}\to \mathbb{R} is called ν-regressive provided 1-\nu (t)p(t)\ne 0 for all t\in {\mathbb{T}}_k. The set of all regressive and ld-continuous functions p:\mathbb{T}\to \mathbb{R} will be denoted by {\mathcal{R}}_{\nu }={\mathcal{R}}_{\nu }(\mathbb{T})={\mathcal{R}}_{\nu }(\mathbb{T},\mathbb{R}). We define the set {\mathcal{R}}_{\nu }^{+}={\mathcal{R}}_{\nu }^{+}(\mathbb{T},\mathbb{R})=\{p\in {\mathcal{R}}_{\nu }:1-\nu (t)p(t)>0,\mathrm{\forall }t\in \mathbb{T}\}.

Definition 2.3 ([42])

If r is a regressive function, then the generalized exponential function {\hat{e}}_r is defined by

for all s,t\in \mathbb{T}, where the ν-cylinder transformation is as in

Lemma 2.1 ([42])

Assume that p,q:\mathbb{T}\to \mathbb{R} are two ν-regressive functions, then

1. (i)

   {\hat{e}}_0(t,s)\equiv 1 and {\hat{e}}_p(t,t)\equiv 1;

2. (ii)

   {\hat{e}}_p(\varrho (t),s)=(1-\nu (t)p(t)){\hat{e}}_p(t,s);

3. (iii)

   {\hat{e}}_p(t,s)=\frac{1}{{\hat{e}}_p(s,t)}=e_{{\ominus }_{\nu }p}(s,t);

4. (iv)

   {\hat{e}}_p(t,s){\hat{e}}_p(s,r)={\hat{e}}_p(t,r);

5. (v)

   {({\hat{e}}_{{\ominus }_{\nu }p}(t,s))}^{\mathrm{\nabla }}=({\ominus }_{\nu }p)(t){\hat{e}}_{{\ominus }_{\nu }p}(t,s).

Lemma 2.2 ([43])

For each t_0\in \mathbb{T} in \mathbb{T}\mathrm{\setminus }{\mathbb{T}}_k the single-point set \{t_0\} is ∇-measurable and its ∇-measure is given by {\mu }_{\mathrm{\nabla }}(\{t_0\})=t_0-\varrho (t_0).

Lemma 2.3 ([43])

If a,b\in \mathbb{T} and a\le b, then

If a,b\in \mathbb{T}\mathrm{\setminus }{\mathbb{T}}^k and a\le b, then

For more details of time scales and ∇-measurability, one is referred to [42, 43]. For more on time scales, see [44–49].

Definition 2.4 ([32–34])

A time scale \mathbb{T} is called an almost periodic time scale if

Remark 2.4 Definition 3.1 introduced in [35] is the same as the concept of almost periodic time scales proposed in [32, 34], and \mathbb{T} is also called an invariant time scale under translations in [35].

After these preparations, in the next section, we will introduce the concept of weighted piecewise pseudo almost automorphic functions on time scales in a Banach space and some of their basic properties are investigated.

In the following, we will give the definition of ld-piecewise continuous functions on time scales.

Definition 3.1 We say \phi :\mathbb{T}\to \mathbb{X} is ld-piecewise continuous with respect to a sequence \{{\tau }_i\}\subset \mathbb{T} which satisfy , i\in \mathbb{Z}, if \phi (t) is continuous on {({\tau }_i,{\tau }_{i+1}]}_{\mathbb{T}} and ld-continuous on \mathbb{T}\mathrm{\setminus }\{{\tau }_i\}. Furthermore, {({\tau }_i,{\tau }_{i+1}]}_{\mathbb{T}} are called intervals of continuity of the function \phi (t).

For convenience, {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) denotes the set of all ld-piecewise continuous functions with respect to a sequence \{{\tau }_i\}, i\in \mathbb{Z}. Similar to Definition 3.1, we can also introduce the concept of functions which belong to {\mathit{PC}}_{\mathrm{rd}}(\mathbb{T},\mathbb{X}).

Throughout the paper, we denote by \mathbb{X} a Banach space; let \mathfrak{B} be the set consisting of all sequences {\{t_i\}}_{i\in \mathbb{Z}} such that \theta ={inf}_{i\in \mathbb{Z}}(t_{i+1}-t_i)>0. For {\{t_i\}}_{i\in \mathbb{Z}}\in \mathfrak{B}, let {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) be the space formed by all bounded ld-piecewise continuous functions \varphi :\mathbb{T}\to \mathbb{X} such that \varphi (\cdot ) is continuous at t for any t\notin {\{t_i\}}_{i\in \mathbb{Z}} and \varphi (t_i)=\varphi (t_i^{-}) for all i\in \mathbb{Z}; let Ω be a subset of \mathbb{X} and let {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T}\times \mathrm{\Omega },\mathbb{X}) be the space formed by all bounded piecewise continuous functions \varphi :\mathbb{T}\times \mathrm{\Omega }\to \mathbb{X} such that, for any x\in \mathrm{\Omega }, \varphi (\cdot ,x)\in {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T}\times \mathbb{X},\mathbb{X}). For any t\in \mathbb{T}, \varphi (t,\cdot ) is continuous at x\in \mathrm{\Omega }.

Let \mathit{UPC}(\mathbb{T},\mathbb{X}) be the space of all functions \phi \in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) such that ϕ satisfies the condition: for any \epsilon >0, there exists a positive number \delta =\delta (\epsilon ) such that if the points t^{\prime }, t^{″} belong to the same interval of continuity of φ and implies .

Now, we introduce the set

which denotes all unbounded increasing sequences of real numbers. Let T,P\in \mathfrak{B} and let s(T\cup P):\mathfrak{B}\to \mathfrak{B} be a map such that the set s(T\cup P) forms a strictly increasing sequence. For D\subset \mathbb{R} and \epsilon >0, we introduce the notations {\theta }_{\epsilon }(D)=\{t+\epsilon :t\in D\}, F_{\epsilon }(D)={\bigcap }_{\epsilon }\{{\theta }_{\epsilon }(D)\}. Denote by \tilde{\varphi }=(\phi (t),T) the element from the space {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X})\times \mathfrak{B}. For every sequence of real numbers \{s_n\}, n=1,2,\dots with {\theta }_{s_n}\tilde{\varphi }:=(\phi (t+s_n),T-s_n), we shall consider the sets \{\phi (t+s_n),T-s_n\}\subset {\mathit{PC}}_{\mathrm{ld}}\times \mathfrak{B}, where

Definition 3.2 Let \{t_i\}\in \mathfrak{B}, i\in \mathbb{Z}. We say \{t_i^j\} is a derivative sequence of \{t_i\} and

Definition 3.3 Let t_i^j=t_{i+j}-t_i, i,j\in \mathbb{Z}. We say \{t_i^j\}, i,j\in \mathbb{Z}, is equipotentially almost automorphic on an almost periodic time scale \mathbb{T} if, for any sequence \{s_n\}\subset \mathbb{Z}, there exists a subsequence \{s_n^{\prime }\} such that

is well defined for each k\in \mathbb{Z} and

for each k\in \mathbb{Z}.

Definition 3.4 A function \varphi \in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) is said to be ld-piecewise almost automorphic if the following conditions are fulfilled:

1. (i)

   T=\{t_k\} is an equipotentially almost automorphic sequence.

2. (ii)

   Let \phi \in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) be a bounded function with respect to a sequence T=\{t_k\}. We say that φ is piecewise almost automorphic if from every sequence {\{s_n\}}_{n=1}^{\mathrm{\infty }}\subset \mathrm{\Pi }, we can extract a subsequence {\{{\tau }_n\}}_{n=1}^{\mathrm{\infty }} such that

   {\tilde{\varphi }}^{\ast }=({\phi }^{\ast }(t),T^{\ast })=\underset{n\to \mathrm{\infty }}{lim}(\phi (t+{\tau }_n),T-{\tau }_n)=\underset{n\to \mathrm{\infty }}{lim}{\theta }_{{\tau }_n}\tilde{\varphi }

is well defined for each t\in \mathbb{T} and

for each t\in \mathbb{T}. Denote by \mathit{AA}(\mathbb{T},\mathbb{X}) the set of all such functions.

1. (iii)

   A bounded function f\in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T}\times \mathbb{X},\mathbb{X}) with respect to a sequence T=\{t_k\} is said to be piecewise almost automorphic if f(t,x) is piecewise automorphic in t\in \mathbb{T} uniformly in x\in B, where B is any bounded subset of \mathbb{X}. Denote by \mathit{AA}(\mathbb{T}\times \mathbb{X},\mathbb{X}) the set of all such functions.

Similarly, we can also introduce the concept of piecewise almost automorphic functions which belong to {\mathit{PC}}_{\mathrm{rd}}(\mathbb{T},\mathbb{X}).

Let U be the set of all functions \rho :\mathbb{T}\to (0,\mathrm{\infty }) which are positive and locally ∇-integrable over \mathbb{T}. For a given r\in [0,\mathrm{\infty })\cap \mathrm{\Pi } and \mathrm{\forall }{t}_0\in \mathbb{T}, set

for each \rho \in U.

Remark 3.1 In (2), if \mathbb{T}=\mathbb{R}, t_0=0, one can easily get

if \mathbb{T}=\mathbb{Z}, t_0=0, one has the following:

Define

It is clear that U_B\subset U_{\mathrm{\infty }}\subset U. Now for \rho \in U_{\mathrm{\infty }} define

Similarly, we define

We are now ready to introduce the sets \mathit{WPAA}(\mathbb{T},\rho ) and \mathit{WPAA}(\mathbb{T}\times \mathbb{X},\rho ) of weighted pseudo almost periodic functions:

Lemma 3.2 Let \varphi \in {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}). Then \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ) where \rho \in U_B if and only if, for every \epsilon >0,

where r\in \mathrm{\Pi } and M_{r,\epsilon ,t_0}(\varphi ):=\{t\in {[t_0-r,t_0+r]}_{\mathbb{T}}:\parallel \varphi (t)\parallel \ge \epsilon \}.

Proof (a) Necessity. For contradiction, suppose that there exists {\epsilon }_0>0 such that

Then there exists \delta >0 such that, for every n\in \mathbb{N},

So we get

where \gamma ={inf}_{s\in \mathbb{T}}\rho (s). This contradicts the assumption.

1. (b)

   Sufficiency. Assume that {lim}_{r\to \mathrm{\infty }}\frac{1}{m(r,\rho ,t_0)}{\mu }_{\mathrm{\nabla }}(M_{r,\epsilon ,t_0}(\varphi ))=0. Then for every \epsilon >0, there exists r_0>0 such that, for every r>r_0,

where and .

Now, we have

Therefore, {lim}_{r\to \mathrm{\infty }}\frac{1}{m(r,\rho ,t_0)}{\int }_{t_0-r}^{t_0+r}\parallel \varphi (s)\parallel \rho (s)\mathrm{\nabla }s=0, that is, \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ). This completes the proof. □

Lemma 3.3 {\mathit{PAA}}_0(\mathbb{T},\rho ) is a translation invariant set of {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) with respect to Π if \rho \in U_B, i.e., for any s\in \mathrm{\Pi }, one has \varphi (t+s):={\theta }_s\varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ) if \rho \in U_B.

Proof For any s\in \mathrm{\Pi }, \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ), \epsilon >0, r>0, we have

Hence

Since \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ), by Lemma 3.2, we have

Furthermore, {lim}_{r\to \mathrm{\infty }}\frac{m(r+|s|,\rho ,t_0)}{m(r,\rho ,t_0)}=1, thus

Again, using Lemma 3.2, one can get {\theta }_s\varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ) for any s\in \mathrm{\Pi }. This completes the proof. □

By Definition 3.4, one can easily get the following lemma.

Lemma 3.4 Let \varphi \in \mathit{AA}(\mathbb{T},\mathbb{X}), then the range of ϕ, \varphi (\mathbb{T}), is a relatively compact subset of \mathbb{X}.

Lemma 3.5 If f=g+\varphi with g\in \mathit{AA}(\mathbb{T},\mathbb{X}), and \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ), where \rho \in U_B, then g(\mathbb{T})\subset \overline{f(\mathbb{T})}.

Proof (1) For any t\in \mathbb{T}\mathrm{\setminus }\{t_i\}, g(t)\in g(\mathbb{T}), one has g(t)=f(t)-\varphi (t). Since g\in \mathit{AA}(\mathbb{T},\mathbb{X}), there exists a sequence \{{\alpha }_n\}\subset \mathrm{\Pi } such that g(t+{\alpha }_n)\to g(t), n\to \mathrm{\infty }.

Furthermore, by Lemma 3.3, \varphi (t+{\alpha }_n)\in {\mathit{PAA}}_0(\mathbb{T},\mathbb{X}), so there exists \beta \in \mathrm{\Pi } such that \varphi (t+{\alpha }_n+\beta )\to 0, n\to \mathrm{\infty }. Hence, let s=t+\beta, and one has

i.e. f(s+{\alpha }_n-\beta )\to g(t) for each t\in \mathbb{T} as n\to \mathrm{\infty }.

1. (2)

   If \{t_i\}\in \mathfrak{B}, noting that Definition 3.4, the above sequence \{{\alpha }_n\}\subset \mathrm{\Pi } and the number \beta \in \mathrm{\Pi } is suitable for the increasing sequence \{t_i\}, so the proof process is the same as (1). This completes the proof. □

Lemma 3.6 The decomposition of a weighted piecewise pseudo almost automorphic function according to \mathit{AA}\oplus {\mathit{PAA}}_0 is unique for any \rho \in U_B.

Proof Assume that f=g_1+{\varphi }_1 and f=g_2+{\varphi }_2. Then (g_1-g_2)+({\varphi }_1-{\varphi }_2)=0. Since g_1-g_2\in \mathit{AA}(\mathbb{T},\mathbb{X}), and {\varphi }_1-{\varphi }_2\in {\mathit{PAA}}_0(\mathbb{T},\rho ), in view of Lemma 3.5, we deduce that g_1-g_2=0. Consequently, {\varphi }_1-{\varphi }_2=0, i.e. {\varphi }_1={\varphi }_2. This completes the proof. □

Theorem 3.7 For \rho \in U_B, (\mathit{WPAA}(\mathbb{T},\rho ),{\parallel \cdot \parallel }_{\mathrm{\infty }}) is a Banach space.

Proof Assume that {\{f_n\}}_{n\in \mathbb{N}} is a Cauchy sequence in \mathit{WPAA}(\mathbb{T},\rho ). We can write uniquely f_n=g_n+{\varphi }_n. Using Lemma 3.5, we see that {\parallel g_p-g_q\parallel }_{\mathrm{\infty }}\le {\parallel f_p-f_q\parallel }_{\mathrm{\infty }}, from which we deduce that {\{g_n\}}_{n\in \mathbb{N}} is a Cauchy sequence in \mathit{AA}(\mathbb{T},\mathbb{X}). Hence, {\varphi }_n=f_n-g_n is a Cauchy sequence in {\mathit{PAA}}_0(\mathbb{T},\rho ). We deduce that g_n\to g\in \mathit{AA}(\mathbb{T},\mathbb{X}), {\varphi }_n\to \varphi \in {\mathit{PAA}}_0(\mathbb{T},\rho ), and finally f_n\to g+\varphi \in \mathit{WPAA}(\mathbb{T},\rho ). This completes the proof. □

Definition 3.5 Let {\rho }_1,{\rho }_2\in U_{\mathrm{\infty }}. One says that {\rho }_1 is equivalent to {\rho }_2, written {\rho }_1\sim {\rho }_2 if {\rho }_1/{\rho }_2\in U_B.

Theorem 3.8 Let {\rho }_1,{\rho }_2\in U_{\mathrm{\infty }}. If {\rho }_1\sim {\rho }_2, then \mathit{WPAA}(\mathbb{T},{\rho }_1)=\mathit{WPAA}(\mathbb{T},{\rho }_2).

Proof Assume that {\rho }_1\sim {\rho }_2. There exist a,b>0 such that a{\rho }_1\le {\rho }_2\le b{\rho }_1. So

where r\in \mathrm{\Pi } and

This completes the proof. □

Lemma 3.9 If g\in \mathit{AA}(\mathbb{T}\times \mathbb{X},\mathbb{X}) and \alpha \in \mathit{AA}(\mathbb{T},\mathbb{X}), then G(t):=g(\cdot ,\alpha (\cdot ))\in \mathit{AA}(\mathbb{T},\mathbb{X}).

Proof Let T=\{t_i\}, \tilde{\varphi }=(g(t,x),T)\in \mathit{AA}(\mathbb{T}\times \mathbb{X},\mathbb{X})\times \mathfrak{B}, from every sequence {\{s_n\}}_{n=1}^{\mathrm{\infty }}\subset \mathrm{\Pi }, we can extract a subsequence {\{{\tau }_n\}}_{n=1}^{\mathrm{\infty }} such that

uniformly exists on {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T}\times \mathbb{X},\mathbb{X})\times \mathfrak{B}. Since \alpha \in \mathit{AA}(\mathbb{T},\mathbb{X}), one can extract \{{\tau }_n^{\prime }\}\subset \{{\tau }_n\} such that

Hence, G\in \mathit{AA}(\mathbb{T},\mathbb{X}). This completes the proof. □

Theorem 3.10 Let f=g+\varphi \in \mathit{WPAA}(\mathbb{T}\times \mathbb{X},\rho ), where g\in \mathit{AA}(\mathbb{T}\times \mathbb{X},\mathbb{X}), \varphi \in {\mathit{PAA}}_0(\mathbb{T}\times \mathbb{X},\rho ), \rho \in U_B, and the following conditions hold:

1. (i)

   \{f(t,x):t\in \mathbb{T},x\in K\} is bounded for every bounded subset K\subseteq \mathrm{\Omega }.

2. (ii)

   f(t,\cdot ), g(t,\cdot ) are uniformly continuous in each bounded subset of Ω uniformly in t\in \mathbb{T}.

Then f(\cdot ,h(\cdot ))\in \mathit{WPAA}(\mathbb{T},\rho ) if h\in \mathit{WPAA}(\mathbb{T},\rho ) and h(\mathbb{T})\subset \mathrm{\Omega }.

Proof We have f=g+\varphi, where g\in \mathit{AA}(\mathbb{T}\times \mathbb{X},\mathbb{X}) and \varphi \in {\mathit{PAA}}_0(\mathbb{T}\times \mathbb{X},\rho ) and h={\varphi }_1+{\varphi }_2, where {\varphi }_1\in \mathit{AA}(\mathbb{T},\mathbb{X}) and {\varphi }_2\in {\mathit{PAA}}_0(\mathbb{T},\rho ). Hence, the function f(\cdot ,h(\cdot )) can be decomposed as

By Lemma 3.9, g(\cdot ,{\varphi }_1(\cdot ))\in \mathit{AA}(\mathbb{T},\mathbb{X}). Now, consider the function

Clearly, \mathrm{\Psi }\in {\mathit{BPC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}). For Ψ to be in {\mathit{PAA}}_0(\mathbb{T},\rho ), it is sufficient to show that

Let K be a bounded subset of Ω such that \varphi (\mathbb{T})\subseteq K, {\varphi }_1(\mathbb{T})\subseteq K. By (ii), f(t,\cdot ) is uniformly continuous in \varphi (\mathbb{T}) uniformly in t\in \mathbb{T}, and we see that, for given \epsilon >0, there exists \delta >0 such that y_1,y_2\in K and implies that

Thus, for each t\in \mathbb{T}, implies for all t\in \mathbb{T},

where {\varphi }_2(t)=h(t)-{\varphi }_1(t). For r>0 and any fixed t_0\in \mathbb{T}, let M_{r,\delta ,t_0}({\varphi }_2)=\{t\in {[t_0-r,t_0+r]}_{\mathbb{T}}:\parallel {\varphi }_2\parallel \ge \delta \}, we can obtain

Now since {\varphi }_2\in {\mathit{PAA}}_0(\mathbb{T},\rho ), Lemma 3.2 yields

and this implies that \mathrm{\Psi }\in {\mathit{PAA}}_0(\mathbb{T},\rho ).

Finally, we need to show \varphi (\cdot ,{\varphi }_1(\cdot ))\in {\mathit{PAA}}_0(\mathbb{T},\rho ). Note that f=g+\varphi and g(t,\cdot ) is uniformly continuous in {\varphi }_1(\mathbb{T}) uniformly in t\in \mathbb{T}. By the assumption (ii), f(t,\cdot ) is uniformly continuous in {\varphi }_1(\mathbb{T}) uniformly in t\in \mathbb{T}, so is ϕ. Since {\varphi }_1(\mathbb{T}) is relatively compact in \mathbb{X}, for \epsilon >0, there exists \delta >0 such that {\varphi }_1(\mathbb{T})\subset {\bigcup }_{k=1}^m{B}_k, where for some x_k\in {\varphi }_1(\mathbb{T}) and

It is easy to see that the set U_k:=\{t\in \mathbb{T}:{\varphi }_1(t)\in B_k\} is open and {\varphi }_1(\mathbb{T})={\bigcup }_{k=1}^m{U}_k. Define

Then it is clear that V_i\cap V_j\ne \mathrm{\varnothing } if i\ne j, 1\le i,j\le m. So we get

In view of (3), it follows that

Thus we get

Since \varphi \in {\mathit{PAA}}_0(\mathbb{T}\times \mathbb{X},\rho ) and {lim}_{r\to \mathrm{\infty }}\frac{1}{m(r,\rho ,t_0)}{\mu }_{\mathrm{\nabla }}(M_{r,\epsilon ,t_0}(\varphi (t,x_k)))=0, it follows that

by Lemma 3.2, \varphi (\cdot ,{\varphi }_1(\cdot ))\in {\mathit{PAA}}_0(\mathbb{T},\rho ). This completes the proof. □

Theorem 3.10 has the following consequence.

Corollary 3.11 Let f=g+\varphi \in \mathit{WPAP}(\mathbb{T},\rho ), where \rho \in U_B. Assume that f and g are Lipschitzian in x\in \mathbb{X} uniformly in t\in \mathbb{T}. Then f(\cdot ,h(\cdot ))\in \mathit{WPAA}(\mathbb{T},\rho ) if h\in \mathit{WPAA}(\mathbb{T},\rho ).

Next, we will show the following two lemmas, which are useful in the proof of our results.

Lemma 3.12 If \phi \in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) is an almost automorphic function with respect to the sequence T and \{t_k\}\subset \mathbb{T} is equipotentially almost automorphic satisfying {inf}_{i\in \mathbb{Z}}{t}_i^q=\theta >0, q\in \mathbb{Z}, then \{\phi (t_k)\} is an almost automorphic sequence in \mathbb{X}.

Proof Let t_i^j=t_{i+j}-t_i, i,j\in \mathbb{Z}. Obviously, from the definition of Π, it is easy to know that t_i^j\in \mathrm{\Pi }. Since \phi \in {\mathit{PC}}_{\mathrm{ld}}(\mathbb{T},\mathbb{X}) is an almost automorphic function and \{t_k\}\subset \mathbb{T} is equipotentially almost automorphic, from Definition 3.3 and Definition 3.4, for any sequence \{s_n\}\subset \mathbb{Z}, we find that there exists a subsequence \{s_n^{\prime }\} such that

and

Hence, \{\phi (t_k)\} is an almost automorphic sequence in \mathbb{X}. This completes the proof. □

Lemma 3.13 A necessary and sufficient condition for a bounded sequence \{a_n\} to be in {\mathit{PAA}}_0(\mathbb{Z},\rho ) is that there exists a uniformly continuous function f\in {\mathit{PAA}}_0(\mathbb{T},\rho ) such that f(t_n)=a_n, t_n\in \mathbb{T}, n\in \mathbb{Z}, \rho \in U_B.

Proof Necessity. We define a function

where r\in \mathrm{\Pi }. It is obviously uniformly continuous on \mathbb{T}. f\in {\mathit{PAA}}_0(\mathbb{T},\rho ) since