Research Article | Open Access

Yurong Li, Jiong Sun, Siqin Yao, "A Class of Generalized Differential Operators with Infinitely Many Discontinuous Points", *Abstract and Applied Analysis*, vol. 2014, Article ID 947182, 8 pages, 2014. https://doi.org/10.1155/2014/947182

# A Class of Generalized Differential Operators with Infinitely Many Discontinuous Points

**Academic Editor:**Valery Serov

#### Abstract

We define a class of differential operators which have infinitely many discontinuous points, and investigate the kernel of in terms of operator theory. It is shown that the solutions of have exponential behavior, and the dimension of the kernel is given.

#### 1. Introduction and Motivations

In the study of ergodicity of billiard flows, a very important question concerns the hyperbolicity of these flows [1, 2]. Hyperbolicity is defined in the language of the linearized system; this is the situation where trajectories close to a given trajectory either diverge exponentially in time from it or converge in the same way. The differential operator we study is associated with the linearization of the flow; hence we ask whether solutions to have the corresponding property. However, this is a paper not about ergodic theory, but about the spectral theory [3, 4] of a class of generalized differential operators which comes up naturally in this context.

The first class of chaotic billiards was introduced by Sinai in [5]; he proved the ergodicity of plenary dispersing billiards. It took more than 30 years; until 2003, BĂˇlint et al. were able to prove ergodicity for multidimensional dispersing billiard in [6]. However it remains an important and difficult question to study hyperbolicity for nondispersing chaotic billiards, as well as high dimensional Bunimovich type billiards. We plan to give a self-contained approach using spectral theory to study the asymptotic behavior of functions in the kernels of a class of ordinary differential operators, motivated by asymptotic behavior of Jacobian filed of certain dynamical systems with singularities, especially billiards.

In this paper we construct a new class of generalized differential operators associated with the impulsive equations. The differential operators we deal with are second order, matrix coefficient SchrĂ¶dinger operators with infinitely many discontinuous points. These kinds of operators are more general than those occurring in billiard flow, but include these as a special case. In this case, the jump conditions correspond to the reflections. We investigate the exponential behavior of functions in the null-space of in terms of operator theory. On this basis, we obtain the relation between the minimal and maximal operators associated with the weighted operators and then characterize the dimension of the kernel. The results of this paper extend the result in the papers of Kauffman and Zhang [7] and Zhang and Lian [8] to more general case, which gives some hope that the structure of the differential operators may be used later to analyze some of the problems of greater interest in multidimensional billiards.

This paper is divided into five sections: the first section introduces the research background; the second section describes the main results derived in this paper; they are formulated in Theorems 7 and 8; in the third section, we studied the differential operator ; then, in the last two sections, we give the proof of the two main theorems.

#### 2. Statement of the Main Results

Throughout this paper, we will let be a partition of , . Denote , for all . Denote as the inner product of vectors . Let be a real interval; denote by the Hilbert space of all measurable functions from to , such that with inner product

*Definition 1. *Let be the differential expression on an interval defined by
where is a positive semidefinite symmetric real matrix for each and a continuous function of .

*Definition 2. *Let be a set of all sequences of complex numbers , which satisfy the condition . In this paper, every element of the sequence is a -dimensional column vector; we denote as a set, which satisfy the following condition: ; then () and , for .

*Definition 3. *Let denote the set of all measurable functions from to , such that is differentiable (in distribution sense) almost everywhere. For any , and the partition of , define the operator:
where and is a matrix.

*Definition 4. *Let
be a linear operator defined as
with domain
where , and denotes the set of absolutely continuous complex valued functions on , and, for each ,
and the following holds:(1), where is an identity matrix;(2) is a symmetric operator on the ;(3)there exists a universal constant , such that for any unit vector , any , and any ,

*Remark 5. *In Definition 4, (8) represents the jump conditions at points . The range of is in , as one can check that

*Definition 6 ( condition). *Let , . be a set of all vectors such that
For any , we say that if there exists a subsequence with
such that for all , .

Note that any function has a certain nice property that for a subsequence which are not too close to each other.

The main results we derived are Theorems 7 and 8.

Theorem 7. *Let , and there exist , such that for interval . Then one has the following.*(1)*If is nonincreasing, then there exists , such that, for all and ,
*(2)*If is nondecreasing, then there exists , such that, for all and ,
*

Next theorem characterizes the dimension of the kernel of .

Theorem 8. *There exist exactly linearly independent bounded solutions to the equation .*

*Example 9. *Consider the one-dimensional case; that is, . Let , be a real number, and let be a continuous function of and satisfy the following conditions: , , s.t., ; then the equation becomes
It describes the movement of the billiard particle in a smooth table; when it reaches the border, its position is not changed, but the direction is changed. Theorems 7 and 8 hold for this special equation; that is, the solutions of (18) have exponential behavior, and there exists exactly one linearly independent bounded solution.

#### 3. Characterization of the Generalized Differential Operator

In this section, we give a characterization on the generalized differential operator .

*Definition 10. *Define an operator , such that
where, for and are nonsingular matrices with , for some , with

Lemma 11. * is surjective.*

*Proof. * can be written as
Since and are nonsingular matrices, so is an invertible matrix. For any , let . Then we can check that and .

In order to study the operator , we first introduce an operator which was studied in [7, 9] and review some properties of .

*Definition 12. *Let be a differential operator defined by
satisfying the following boundary conditions: for any ,
where , are the same as in the definition of .

Lemma 13 (see [7]). *Let be the maximal operator of , and let be the minimal operator. Then the adjoint operator of satisfies
*

Lemma 14 (see [7]). *For all in the domain of ,
**
where is a universal positive constant.*

Lemma 15 (see [9]). *Let be a densely defined closed operator on a Hilbert space. Denote the range of by and the nullspace of by . Then
*

Based on all the above properties for operator , we obtain the following result.

Lemma 16. *Let . Then there exists such that
*

*Proof. *By Lemma 14, we know that . From Lemmas 13 and 15, it is easy to show that . So is surjective. Then we have, for all , , such that . But by the definition of , we know that , which gives the result.

Lemma 17. *Assuming that there exists , such that
**
then the range of is a subset of .*

*Proof. *Since and , so
There exists , such that, on each ,
Therefore , , , and are all in . It follows that
Thus the range of is contained in .

Next we recall Gronwallâ€™s inequality, which will be used in our proof that is subjective below.

Lemma 18 (Gronwallâ€™s inequality). *Let be continuous on and satisfy
**
where and is continuous on . Then
**
for .*

Proposition 19. * is surjective.*

*Proof. *Let . By Lemma 16, there exists , such that
We just need to find a , such that . By Definition 4, that is,
For each , let be the fundamental solution matrix of
with . Since we assumed that , for all , so there exists a uniform constant , such that . By Lemma 11, we choose , and , then we know that there exist , such that
That is,
Letting satisfy and
then we have
Let
and then we get
It follows that .

In the following, it is enough to show that . On each , we have
Denote
Then
By hypothesis of , we know that , such that and ; thus on each
By Gronwallâ€™s inequality,
Combining all these facts together, we get
Therefore
Thus satisfies
This completes the proof of Proposition 19.

#### 4. Exponential Behavior of Solutions: Proof of Theorem 7

In this section, we give the proof of Theorem 7 by considering the exponential behavior of the null space of .

For any , we denote Note that , so

Lemma 20. *Let for interval . Then there exists a constant , and , such that for any *

*Proof. *Assume that there exists , such that
since and , so
This implies that
Thus
where is the number of , such that . But we know that
so there exists , such that, for any ,
where .

Now we are ready to prove Theorem 7.

*Proof. *By the definition of , we know that
Thus
(1) Since is nonincreasing, then ; by Lemma 20, we have
so
(2) Since is nondecreasing, then ; by Lemma 20, we have
so

#### 5. Proof of Theorem 8

In this section, we give the proof of Theorem 8. First we obtain the relation between the minimal and maximal operators associated with the weighted operators and then characterize the dimension of the kernel of .

*Definition 21. *Let be defined as Definition 12, . Define

*Definition 22. *Let the maximal operator associated with be the restriction of on , with
Let be the set of all , such that , for all and vanishes in a neighborhood of both â€śend pointsâ€ť of and is compact. Denote to be restricting on ; then we define the minimal operator to be the smallest closed operator in which extends and denote by the domain of .

Lemma 23. *If is defined as in Definition 21 on , then
**
and, for any in the domain of , we have . If is compact, say , then for any , also .*

*Proof. *Let be a Cauchy sequence of functions from converging to with converging to in . Then on any compact interval , and both converge uniformly; therefore also converges in each , so converges in . It follows that is absolutely continuous on each , and , if ; then also . Since converges to for , we see that , is in , and so .

*Definition 24 (see [10]). *A densely defined operator on a Hilbert space is said to be symmetric if for all in the domain of .

Lemma 25. * is a symmetric operator, for or .*

*Proof. *For any in the domain of , by Lemma 23, we know that and . Without loss of generality, we suppose that and are real vector valued functions:

If is a compact interval , then by the fact that
we get .

If , by hypotheses on the boundary conditions and , we have
Thus we get
That is, . So by Definition 24, is symmetric.

Lemma 26. *For , the equation has a solution in the domain of if and only if is orthogonal to all solutions of .*

*Proof. *Assume that, for , the equation has a solution . For any , such that , we have
Conversely, if is orthogonal to all solutions of , choose such that and . We need to show that . Using the same calculation as in the proof of Lemma 25, we get
On the other hand, Consider
Thus we get that for any satisfying . Choose , s.t. , but ; then we get ; similarly, we can get .

Corollary 27. *; letting denote the restriction of to domain
**
then .*

Based on all the above results, we can derive the relation between the operators and .

Proposition 28. *If is defined as in Definition 21 and , then
*

*Proof. *Since is the smallest closed extension of , it follows that
As for any (then ) and , , then
Let be in the domain of . Since is surjective for compact , so there is a function in such that ; thus
So if is in the range of with , we have
Therefore is orthogonal to the range of , but, by Lemma 26, is in the null space of . Hence ; that is, .

Proposition 29. *If is defined as in Definition 21 and , then
*

*Proof. *By Proposition 28, we only need to show . If is in the domain of , then, on any compact subinterval of , we have
Thus the restriction of to is in the domain of . But, by Proposition 28, the restriction of to must agree with . Since is arbitrary, the proposition is proved.

We can show that is semibounded in the next lemma.

Lemma 30. *If is defined as in Definition 21, for all ,
*

*Proof. *By integration by parts we obtain, for ,
where the last inequality follows from Hardyâ€™s inequality [11].

In order to prove Theorem 8, we need to introduce the Friedrichs extension of .

*Definition 31. *With as in Definition 21, let , for all .

It follows from operator theory [9] that the semibounded symmetric operator has equal deficiency indices, and therefore, by Von Neumannâ€™s theorem, such an operator always has self-adjoint extensions. There is a distinguished extension , called the Friedrichs extension [11], which is obtained from the quadratic form associated with .

Proposition 32. * is a closable quadratic form and its closure is the quadratic form of a unique self-adjoint operator defined by
**
Furthermore,
*

*Proof. *This is a form of the definition of the Friedrichs extension: for the semibounded symmetric operator and the closable quadratic form , the restriction of to the domain of the closure of is in fact the Friedrichs extension. This form is clearly closable. The last inequality follows from the construction of the Friedrichs extension , together with Lemma 30.

Next proposition tells us the relation between and .

Proposition 33. * is nonincreasing if and only if is a nonincreasing solution to .*

*Proof. *Let ; if is nonincreasing, then . So by east calculation, we have
which implies the result.

Now we are ready to prove Theorem 8.

*Proof. *It is sufficient to consider real solutions. Let be any real solution of ; for any , let , . Using the fact that is positive semidefinite for each , we have
On the other hand, for any ,
This implies that is nondecreasing on . Choosing , such that , then , from (52); is nondecreasing. Then, by Theorem 7, we know that those solutions have exponentially increasing amplitude; that is, they are unbounded solutions. Clearly . The set is dimensional, which means that there are at most linearly independent bounded solutions for .

Now we prove , using the properties of Friedrichs extension of . Let be the Friedrichs extension of ; then, by Proposition 32, is self-adjoint and positive definite. Also for any in , . We choose compactly supported piecewise smooth functions on , such that the space spanned by has dimension . Then are linearly independent mod . It is clear that for ; this forces the Fredholm index to increase by . Since is already surjective, this produces an dimensional nonincreasing solution space to , so ; by Proposition 33, we know that there are at least linearly independent bounded solutions for . This completes the proof.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

The authors thank the referee for her/his suggestions which have improved the presentation of the paper. The work of the authors is supported by the National Nature Science Foundation of China (Grant no. 11161030) and the third author is supported by the Program of Higher Level Talents of Inner Mongolia University (SPH-IMU).

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#### Copyright

Copyright © 2014 Yurong Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.