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Existence of Positive, Negative, and SignChanging Solutions to Discrete Boundary Value Problems
Boundary Value Problemsvolume 2011, Article number: 172818 (2011)
Abstract
By using critical point theory, LyapunovSchmidt reduction method, and characterization of the Brouwer degree of critical points, sufficient conditions to guarantee the existence of five or six solutions together with their sign properties to discrete secondorder twopoint boundary value problem are obtained. An example is also given to demonstrate our main result.
1. Introduction
Let , , and denote the sets of all natural numbers, integers, and real numbers, respectively. For , define , when . is the forward difference operator defined by .
Consider the following discrete secondorder twopoint boundary value problem (BVP for short):
where is a given integer.
By a solution to the BVP (1.1), we mean a real sequence satisfying (1.1). For with , we say that if there exists at least one such that . We say that is positive (and write ) if for all , and : , and similarly, is negative () if for all , and . We say that is signchanging if is neither positive nor negative. Under convenient assumptions, we will prove the existence of five or six solutions to (1.1), which include positive, negative, and signchanging solutions.
Difference BVP has widely occurred as the mathematical models describing reallife situations in mathematical physics, finite elasticity, combinatorial analysis, and so forth; for example, see [1, 2]. And many scholars have investigated difference BVP independently mainly for two reasons. The first one is that the behavior of discrete systems is sometimes sharply different from the behavior of the corresponding continuous systems. For example, every solution of logistic equation is monotone, but its discrete analogue has chaotic solutions; see [3] for details. The second one is that there is a fundamental relationship between solutions to continuous systems and the corresponding discrete systems by employing discrete variable methods [1]. The classical results on difference BVP employs numerical analysis and features from the linear and nonlinear operator theory, such as fixed point theorems. We remark that, usually, the application of the fixed point theorems yields existence results only.
Recently, however, a few scholars have used critical point theory to deal with the existence of multiple solutions to difference BVP. For example, in 2004, Agarwal et al. [4] employed the mountain pass lemma to study (1.1) with and obtained the existence of multiple solutions. Very recently, Zheng and Zhang [5] obtained the existence of exactly three solutions to (1.1) by making use of threecriticalpoint theorem and analytic techniques. We also refer to [6–9] for more results on the difference BVP by using critical point theory. The application of critical point theory to difference BVP represents an important advance as it allows to prove multiplicity results as well.
Here, by using critical point theory again, as well as LyapunovSchmidt reduction method and degree theory, a sharp condition to guarantee the existence of five or six solutions together with their sign properties to (1.1) is obtained. And this paper offers, to the best of our knowledge, a new method to deal with the sign of solutions in the discrete case.
Here, we assume that and
Hence, grows asymptotically linear at infinity.
The solvability of (1.1) depends on the properties of both at zero and at infinity. If
where is one of the eigenvalues of the eigenvalue problem
then we say that (1.1) is resonant at infinity (or at zero); otherwise, we say that (1.1) is nonresonant at infinity (or at zero). On the eigenvalue problem (1.4), the following results hold (see [1] for details).
Proposition 1.1.
For the eigenvalue problem (1.4), the eigenvalues are
and the corresponding eigenfunctions with are .
Remark 1.2.

(i)
The set of functions is orthogonal on with respect to the weight function ; that is,
(1.6)
Moreover, for each .

(ii)
It is easy to see that is positive and changes sign for each ; that is, : and : for .
The main result of this paper is as follows.
Theorem 1.3.
If with , and , then (1.1) has at least five solutions. Moreover, one of the following cases occurs:

(i)
is even and (1.1) has two signchanging solutions,

(ii)
is even and (1.1) has six solutions, three of which are of the same sign,

(iii)
is odd and (1.1) has two sighchanging solutions,

(iv)
is odd and (1.1) has three solutions of the same sign.
Remark 1.4.
The assumption in Theorem 1.3 is sharp in the sense that when for , Theorem 1.4 of [5] gives sufficient conditions for (1.1) to have exactly three solutions with some restrictive conditions.
Example 1.5.
Consider the BVP
where is defined as follows:
It is easy to verify that , , and . So, all the conditions in Theorem 1.3 are satisfied with . And hence (1.7) has at least five solutions, among which two signchanging solutions or three solutions of the same sign.
By the computation of critical groups, for , we have the following.
Corollary 1.6 (see Remark 3.7 below).
If , and , then (1.1) has at least one positive solution and one negative solution.
2. Preliminaries
Let
Then, is a dimensional Hilbert space with inner product
by which the norm can be induced by
Here, denotes the Euclidean norm in , and denotes the usual inner product in .
Define
Then, the functional is of class with
So, solutions to (1.1) are precisely the critical points of in .
As we have mentioned, we will use critical point theory, LyapunovSchmidt reduction method, and degree theory to prove our result. Let us collect some results that will be used below. One can refer to [10–12] for more details.
Let be a Hilbert space and . Denote
for . The following is the definition of the PalaisSmale (PS) compactness condition.
Definition 2.1.
The functional satisfies the (PS) condition if any sequence such that is bounded and as has a convergent subsequence.
In [13], Cerami introduced a weak version of the (PS) condition as follows.
Definition 2.2.
The functional satisfies the Cerami (C) condition if any sequence such that is bounded and , as has a convergent subsequence.
If satisfies the (PS) condition or the (C) condition, then satisfies the following deformation condition which is essential in critical point theory (cf. [14, 15]).
Definition 2.3.
The functional satisfies the () condition at the level if for any and any neighborhood $\mathcal{N}$ of , there are and a continuous deformation such that

(i)
for all ,

(ii)
for all ,

(iii)
is nonincreasing in for any ,

(iv)
.
satisfies the (D) condition if satisfies the () condition for all .
Let denote singular homology with coefficients in a field . If is a critical point of with critical level , then the critical groups of are defined by
Suppose that is strictly bounded from below by and that satisfies () for all . Then, the th critical group at infinity of is defined in [16] as
Due to the condition , these groups are not dependent on the choice of .
Assume that and satisfies the (D) condition. The Morsetype numbers of the pair are defined by , and the Betti numbers of the pair are defined by . By Morse theory [10, 11], the following relations hold:
It follows that for all . If , then for all . Thus, when for some must have a critical point with .
The critical groups of at an isolated critical point describe the local behavior of near , while the critical groups of at infinity describe the global property of . In most applications, unknown critical points will be found from (2.9) or (2.10) if we can compute both the critical groups at known critical points and the critical groups at infinity. Thus, the computation of the critical groups is very important. Now, we collect some useful results on computation of critical groups which will be employed in our discussion.
Proposition 2.4 (see [16]).
Let be a real Hilbert space and . Suppose that splits as such that is bounded from below on and for as . Then for .
Proposition 2.5 (see [17]).
Let be a separable Hilbert space with inner product and corresponding norm , , closed subspaces of such that . Assume that satisfies the (PS) condition and the critical values of are bounded from below. If there is a real number such that for all and , there holds
then there exists a functional : such that
Moreover, if and , then .
Let denote the open ball in about 0 of the radius , and let denote its boundary.
Lemma 2.6 (Mountain Pass Lemma [10, 11]).
Let be a real Banach space and satisfying the (PS) condition. Suppose that and
(J1) there are constants such that , and
(J2) there is a such that .
Then, possesses a critical value . Moreover, can be characterized as
where
Definition 2.7 (Mountain pass point).
An isolated critical point of is called a mountain pass point if .
To compute the critical groups of a mountain pass point, we have the following result.
Proposition 2.8 (see [11]).
Let be a real Hilbert space. Suppose that has a mountain pass point and that is a Fredholm operator with finite Morse index satisfying
Then,
The following theorem gives a relation between the LeraySchauder degree and the critical groups.
Let be a real Hilbert space, and let be a function satisfying the (PS) condition. Assume that , where : is a completely continuous operator. If is an isolated critical point of , that is, there exists a neighborhood of , such that is the only critical point of in , then
where denotes the LeraySchauder degree.
Finally, we state a global version of the LyapunovSchmidt reduction method.
Lemma 2.10 (see [18]).
Let be a real separable Hilbert space. Let and be closed subspaces of such that and . If there are such that for all ,
then the following results hold.
(i) There exists a continuous function : such that
Moreover, is the unique member of such that
(ii) The function : defined by is of class , and
(iii) An element is a critical point of if and only if is a critical point of .
(iv) Let and be the projection onto across . Let and be open bounded regions such that
If for , then
where denotes the LeraySchauder degree.
(v) If is a critical point of mountain pass type of , then is a critical point of mountain pass type of .
3. Proof of Theorem 1.3
In this section, firstly, we obtain a positive solution and a negative solution with to (1.1) by using cutoff technique and the mountain pass lemma. Then, we give a precise computation of . And we remark that under the assumptions of Theorem 1.3, can be completely computed by using Propositions 2.4 and 2.5. Based on these results, four nontrivial solutions to (1.1) can be obtained by (2.9) or (2.10). However, it seems difficult to obtain the sign property of and through their depiction of critical groups. To conquer this difficulty, we compute the Brouwer degree of the sets of positive solutions and negative solutions to (1.1). Finally, the third nontrivial solution to (1.1) is obtained by LyapunovSchmidt reduction method, and its characterization of the local degree results in one or two more nontrivial solutions to (1.1) together with their sign property.
Let
and . The functionals are defined as
Remark 3.1.
From the definitions of and , it is easy to see that if is a critical point of (or ), then (or ).
Lemma 3.2.
The functionals satisfy the (PS) condition; that is, every sequence in such that is bounded, and as has a convergent subsequence.
Proof.
We only prove the case of . The case of is completely similar. Since is finite dimensional, it suffices to show that is bounded. Suppose that is unbounded. Passing to a subsequence, we may assume that and for each , either or is bounded.
Set . For a subsequence, converges to some with . Since for all , we have
Hence,
If , then
where , . If is bounded, then
Letting in (3.4), we have
which implies that satisfies
Because , we see that if is a solution to (3.8), then is positive. Since this contradicts , we conclude that is the only solution to (3.8). A contradiction to .
Lemma 3.3.
Under the conditions of Theorem 1.3, has a positive mountain passtype critical point with has a negative mountain passtype critical point with .
Proof.
We only prove the case of . Firstly, we will prove that satisfies all the conditions in Lemma 2.6. And hence, has at least one nonzero critical point . In fact, , and satisfies the (PS) condition by Lemma 3.2. Clearly, . Thus, we still have to show that satisfies (J1), (J2). To verify (J1), set , then for any , there exists , such that
So, by Taylor series expansion,
Take , then . If we set , then
Since for all , if , then for every and hence
where , . If we take
then . And hence, (J1) holds.
To verify (J2), note that implies that there exist and , such that
So, if we take with , then
So, if we take sufficiently large such that and for , then (J2) holds.
Now, by Lemma 2.6, has at least a nonzero critical point . And for all , we claim that . If not, set , then for all . By for all . Hence, .
In the following, we will compute the critical groups by using Proposition 2.8.
Assume that
and that there exists such that
This implies that satisfies
Hence, the eigenvalue problem
has an eigenvalue . Condition implies that 1 must be a simple eigenvalue; see [1]. So, . Since is finite dimensional, the Morse index of must be finite and must be a Fredholm operator. By Proposition 2.8, . Finally, choose the neighborhood of such that for all , then
The proof is complete.
Lemma 3.4.
By , one has
Proof.
By assumption, we have and for all ,
which implies that 0 is a local minimizer of both and . Hence, (3.21) holds.
Remark 3.5.
Under the conditions of Theorem 1.3, we have
We will use Propositions 2.4 and 2.5 to prove (3.23). Very similar to the proof of Lemma 3.2, we can prove that satisfies the (PS) condition. And it is easy to prove that satisfies (2.11). In fact, let
By , for all and , we have
Hence, if we set , then (2.11) holds.
Now, noticing that implies that there exist , and such that
Hence, we have
Then, (3.23) is proved by Propositions 2.4 and 2.5.
Remark 3.6.
Following the proof of Theorem 3.1 in [17], (3.23) implies that there must exist a critical point of satisfying
It is known that the critical groups are useful in distinguishing critical points. So far, we have obtained four critical points 0, , , and together with their characterization of critical groups. Assume that 0, , , and are the only critical points of . Then, the Morse inequality (2.10) becomes
This is impossible. Thus, must have at least one more critical point . Hence, (1.1) has at least five solutions. However, it seems difficult to obtain the sign property of and . To obtain more refined results, we seek the third nontrivial solution to (1.1) by LyapunovSchmidt reduction method and then its characterization of the local degree results in one or two more nontrivial solutions to (1.1) together with their sign property.
Remark 3.7.
The condition in Theorem 1.3 is necessary to obtain three or more nontrivial solutions to (1.1). In fact, if , then we have
Hence, may coincide with or which becomes an obstacle to seek other critical points by using Morse inequality. If , then
Hence, one cannot exclude the possibility of .
To compute the degree of the set of positive (or negative) solutions to (1.1), we need the following lemma.
Lemma 3.8.
There exists large enough, such that
Proof.
We only prove the case of . For any , define as
Let . The functional : is defined as
It is obvious that is of class and its critical points are precisely solutions to
Since , we see that if is a solution to (3.36), then is positive. Because this contradicts , we conclude that is the only critical point of .
We claim that if is a ball in containing zero, then . In fact, since and for . Hence, for , we have
where we have used the fact that is positive on . Then, for each and , we have
Hence, by invariance under homotopy of Brouwer degree, we have
where .
Now, let . We claim that for large enough and for all , the function has no zero on .
In fact, we have proved that for all and for all , we have
On the other hand, by the definition of , for all , there exists large enough such that for . Since , for , take , then
For , take , then
Hence, if we take , then for , we have , and for , we have . So, if we let
then for all . And for all , we have
So far, we have proved that for large enough, has no zero point on for each . Hence, by invariance under homotopy of Brouwer degree, we obtain
This completes the proof.
Remark 3.9.
By Theorem 2.9 and the above results, we have the following characterization of degree of critical points.
If () is a neighborhood of () containing no other critical points, then
Assume that is a ball centered at zero containing on other critical points, then
Hence, if is a bounded region containing the positive critical points and no other critical points, then by (3.33) we have
Similarly, we see that if is a bounded region containing the negative critical points and no other critical points, then
Now, we can give the proof of Theorem 1.3.
Proof of Theorem 1.3.
The functional satisfies (2.18) in Lemma 2.10 due to the fact that satisfies (2.11). Hence, by Lemma 2.10, there exists such that
Moreover, is the unique member of such that
The function defined by is of class . Because , (3.27) implies that as . Since , there must exist such that . Take , then by (iii) of Lemma 2.10. If is a neighborhood of containing no other critical points of , taking , then . Then, by part (iv) of Lemma 2.10, we have
Suppose that Is Even
Let be large enough so that if , then . Because and is of class , there exists such that for . Because is coercive, . Hence, if we set : , then by (iv) of Lemma 2.10, we have
Suppose that is finite. Let , , and be disjoint open bounded regions in such that is the set of positive critical points of , and is the set of negative critical points of . So far, we have proved that
(i) If , then is sign changing. Let denote an open bounded region disjoint from such that . By the excision property of Brouwer degree, we have
Thus, by Kronecker existence property of Brouwer degree, we see that there must exist such that , which proves that (1.1) has at least five solutions. In this case, both and change sign.

(ii)
Suppose now that . Without loss of generality, we may assume that . Let be a neighborhood of such that . By Lemma 3.3, there exists a critical point of mountain pass type such that if is a neighborhood of such that , then . Thus,
(3.56)
Thus, by Kronecker existence property of Brouwer degree, there exists such that . Finally,
Thus, there must exist such that . Thus, the set together with a critical point of in shows that (1.1) has five nontrivial solutions. Since and , is a signchanging solution, and , , and have the same sign. This completes the proof of Theorem 1.3, when is even.
Suppose that Is Odd

(iii)
Let , , and be as above. If , the proof follows very closely that of the case (i).

(iv)
Suppose that , hence . Because , there exists such that if . So, if , then and is a local maximum of . Since we are assuming (1.1) to have only finitely many solutions, is a strictly local maximum of . Let be such that if . Since , is path connected. Thus, is not a critical point of mountain pass type. By Lemma 3.3, has a critical point of mountain pass type . By (v) of Lemma 2.10, , and hence . Let be neighborhoods of and , respectively, such that and . Thus,
(3.58)
Thus, by Kronecker existence property of Brouwer degree, there exists a third positive solution . So far, we have proved that (1.1) has at least four nontrivial solutions and that have the same sign. This proves Theorem 1.3.
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Acknowledgments
Project supported by National Natural Science Foundation of China (no. 11026059) and Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (no. LYM09105).
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Keywords
 Real Hilbert Space
 Critical Group
 Fredholm Operator
 Morse Index
 Mountain Pass