A class of linear dynamical equations for a Banach space on time scales
© Feng; licensee Springer. 2013
Received: 21 May 2013
Accepted: 18 October 2013
Published: 9 November 2013
In this paper we consider the existence and uniqueness of global solutions to linear dynamical equations for a Banach space on time scales from a new point of view. We characterize those linear dynamical equations for a Banach space whose existence and uniqueness of global solutions do not depend on concrete time scales.
MSC:34G10, 34K30, 39A13.
Keywordstime scales linear dynamical equations existence uniqueness
The calculus of time scales was introduced by Hilger in his PhD dissertation  in order to unify continuous and discrete analysis. In 2001, Bohner and Peterson published a comprehensive introduction to time scales . Another book which consolidates further research in the area is . The reader is referred to [1–9] for better understanding of time scales. We begin with introducing briefly some notation and terminologies.
We denote a time scale, which is a closed subset of the real numbers, by the symbol . Thus ℝ and ℤ, i.e., the real numbers and the integers, are the examples of time scales. The forward jump operator and backward jump operator and the graininess are defined by , and for , respectively (supplemented by and ). A point is called right-dense, right-scattered, left-dense, left-scattered, if , , , hold, respectively. The set which is derived from the time scale is defined as follows: If has a left-scattered maximum m, then . Otherwise, .
Let be a complex Banach space, be the class of all bounded linear operators on . A delta derivative for a function was introduced by Hilger .
Let . Then we define to be the element in (provided it exists) with the property that given any , there is a neighborhood U of t (i.e., for some ) such that for all . In this case, is called the delta (or Hilger) derivative of f at t. Moreover, f is said to be delta (or Hilger) differentiable on if exists for all .
This delta derivative is equal to (the usual derivative) if , and it is equal to △f (the usual forward difference) if . So, the study of dynamical equations on time scales allows a simultaneous treatment of differential and difference equations. And this field has attracted many researchers’ attention.
Especially for linear dynamical equations in finite-dimensional spaces on time scales, a lot of results have been obtained (see [2–4, 10–15]). While for dynamical equations in Banach spaces on time scales, only a few results have been obtained (see [7, 16, 17]). In [7, 17], Hilger obtained the existence and uniqueness conditions for the global solutions to nonlinear and linear dynamical equations, while the conditions are closely dependent on the concrete time scales. It is natural to ask whether or not there exists a class of equations whose existence and uniqueness of global solutions do not depend on the concrete time scales.
where is a time scale, the function y is from to , , is rd-continuous and is rd-continuous in the strong operator topology (SOT).
where denotes the spectrum of A and .
The main result of this paper is the following theorem.
Theorem 1.1 The class is the largest one of those classes ensuring that equation (1) has exactly one global solution for any time scale and any rd-continuous with .
From Theorem 1.1 we obtain the largest class of equations whose existence and uniqueness of global solutions do not depend on the concrete time scales. In Section 2, we make some preparations about time scales and show some needed lemmas. In Section 3, we give the proof of Theorem 1.1.
Firstly, we introduce some needed definitions.
Definition 2.1 
A function is said to be rd-continuous if it is continuous at right-dense points in and its left-sided limits exist at left-dense points in . The set of all rd-continuous functions will be denoted by .
Similarly, the set of all rd-continuous functions in the norm topology will be denoted by .
Definition 2.2 
A function is called if is invertible for all .
Definition 2.3 
rd-continuous if is rd-continuous for any continuous function ;
regressive at if the mapping is invertible (where id is the identity function), and f is called regressive on if f is regressive at each ;
bounded on a set if there exists a constant such that for all .
where is a time scale, the function y is from to , , is a function from .
Lemma 2.4 
If is regressive and rd-continuous on , then equation (2) has exactly one global solution on .
Especially for the linear form of equation (2), we obtain simpler conditions in the following lemma.
The function is regressive on if and only if the function is regressive on .
The function is rd-continuous on if and only if the function is rd-continuous in the strong operator topology and is rd-continuous on .
Proof (i) `⟹’. Assume that is regressive on . Then, for any , is invertible. So is invertible from to and therefore is regressive on .
`⟸’. If the function is regressive on , then for any , is invertible on . And then is invertible from to .
(ii) `⟹’. From Definition 2.3, it is easy to see that is rd-continuous on if we take the continuous function . Similarly, for any fixed , let the function on , then is also rd-continuous. Therefore is rd-continuous on . So the function is rd-continuous in the strong operator topology.
So it suffices to show that for any , there exists such that for any t with , we have .
So for any . By the principle of uniform boundedness , there exists such that . And then from the continuity of . This is a contradiction to inequality (3).
So is continuous on right-dense points. Similarly, we can show that for left-dense points, the left-sided limits of exist. □
It should be noticed that the proof of Lemma 2.5 is independent of special time scale features, except for the notion of rd-continuity at the end. We can obtain the following corollary easily by Lemma 2.4 and Lemma 2.5.
Corollary 2.6 Let be some time scale, be the right-hand side of equation (2). If the function is regressive and rd-continuous in the strong operator topology on , is rd-continuous on , then equation (2) has exactly one global solution on .
Remark 2.7 Corollary 2.6 implies the result for finite-dimensional spaces (see [, Theorem 5.8]) and improves the conclusion for infinite-dimensional spaces (see [, Section 6]) because the strong operator topology is equal to the norm topology in finite-dimensional spaces but weaker than the latter in infinite-dimensional spaces.
Therefore the condition ‘strong operator topology’ in (ii) of Lemma 2.5 cannot be replaced by ‘norm topology’. In fact, we have the following counterexample.
Then is an operator-valued function. We first show that is rd-continuous in strong operator topology.
This implies that is continuous with respect to the SOT in the range space. On the two notions of rd-continuity and continuity coincide.
On the other hand, it is obvious that for any , , is an orthogonal projection on with norm 1. Therefore we conclude that is not rd-continuous in the norm topology.
3 Proof of the main result
From Lemma 2.4 and Corollary 2.6, we can see that existence and uniqueness of the global solution to equation (1) depend closely on the regressivity of the operator-valued function . Now we are going to give the proof of the main theorem.
Proof of Theorem 1.1 We give the proof by two steps.
Step 1. is a class of operators which ensures that equation (1) has a unique solution for any time scale and any with .
We have the graininess . If , then is invertible for any . So is regressive on any time scale. Moreover, we know from equation (1) that is rd-continuous and is rd-continuous in the strong operator topology. From Corollary 2.6, equation (1) has a unique solution on the whole time scale.
Step 2. Suppose that is a class of operators with satisfying condition (i). We need only to prove that there exist a time scale and a function A from to with such that equation (1) either has no solution or has at least two solutions on .
It follows from that is not invertible.
If is not injective, then is not unique.
If is not surjective, then we can choose (where is the range of ). Hence does not exist. This completes the proof. □
Remark 3.1 For being the matrix algebra , it is easy to see that the closure of is equal to . Hence we can deduce that is a very large class of operators in . We can also consider the closure and interior of the class in a Hilbert space in the future.
The author gratefully acknowledges the invaluable advice and encouragement of Professor You Qing Ji. I would like to express my sincere thanks to the reviewers for their helpful comments and advice. Supported by NNSF of China (11271150) and ‘Twelfth Five-Year Plan’ Science and Technology Research Project of the Education of Jilin Province (2013215, 2012186, 2013373, 2011113).
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