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International Journal of Differential Equations
Volume 2014 (2014), Article ID 564914, 8 pages
Resonant Problems by Quasilinearization
Faculty of Natural Sciences and Mathematics, Daugavpils University, Parādes Street 1, Daugavpils 5401, Latvia
Received 29 January 2014; Accepted 6 March 2014; Published 22 April 2014
Academic Editor: Ioannis G. Stratis
Copyright © 2014 Nadezhda Sveikate. 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.
The Dirichlet resonant boundary value problems are considered. If the respective nonlinear equation can be reduced to a quasilinear one with a nonresonant linear part and both equations are equivalent in some domain and if solutions of the quasilinear problem are in , then the original problem has a solution. We say then that the original problem allows for quasilinearization. If quasilinearization is possible for essentially different linear parts, then the original problem has multiple solutions. We give conditions for Emden-Fowler type resonant boundary value problem solvability and consider examples.
Two-point nonlinear boundary value problems often appear in applications. Consider the following one. Let a nonlinear equation be given together with the Dirichlet boundary conditions The left hand side of the equation is the second order linear form where and generally are continuous functions given in the interval . The right side is continuous. A solution is a function in . Even equation of the form and related boundary value problems are investigated insufficiently. The classical result says that the problem (4) and (2) is solvable if is a bounded function. Otherwise various cases are possible. To reduce the problem to that with bounded right hand side the method of lower and upper functions can be used. This method is well developed and related descriptions and other information can be found in the books [1–4]. This method cannot be applied to equations which exhibit oscillatory behavior however. The lower and upper functions in case of the existence of a solution exist but coincide with a solution.
If this is not true the existence of a solution cannot be proved. There are examples, for instance, , and showing that a solution does not exist. This type of problems are called resonant problems.
To treat resonant problems, various approaches were used. We focused on the quasilinearization method [14, 15], which consists in reducing the resonant problem to nonresonant one using suitable estimations of expected solutions.
Below we give a description of the quasilinearization method.
Definition 1. The linear part of differential equation (6) is called a nonresonant with respect to the boundary conditions (7) if the homogeneous problem has only the trivial solution. Otherwise, if the homogeneous problem (8) has nontrivial solutions, then the linear part is called resonant.
For instance, is nonresonant, but is resonant in ; is nonresonant with respect to the boundary conditions , if , ; that is, the coefficient belongs to one of the intervals These intervals are called nonresonant intervals.
The classical result states that the problem (6) and (7) is solvable if linear part is nonresonant and is continuous and bounded (the Picard theorem ). Therefore it is desirable to obtain the conditions for the boundary value problem (6) and (7) to be solvable for resonant linear parts.
In one of the most popular articles on resonant problems  the authors considered the boundary value problem Here , is a constant, is a continuous function, and the limits exist and are finite.
In  the authors formulated the theorem that problem (11) has at least one solution if the inequalities hold. This theorem is known as the Landesman-Lazer condition. Boundary value problem (11) has also been studied by Lazer , Alonso and Ortega , Ahmad , and Cesari and Kannan .
Our main result in this paper is to get conditions which guarantee that the resonant boundary value problem is solvable and we do not use the Landesman-Lazer condition.
We use the quasilinearization approach. This approach was developed in [14, 15]. Using this approach we reduce (6) to a quasilinear one of the form where a function is continuous and bounded by a constant and the linear part is nonresonant yet with respect to the given boundary conditions (7) that means that the respective homogeneous problem has only the trivial solution.
Our paper consists of the introduction, four sections, conclusions and references.
Definitions of the type of a solution to Dirichlet boundary value problems are given in Section 2. The general result and related auxiliary results for quasilinear problems are stated.
In Section 4 the idea of quasilinearization is applied to the investigation of the Emden-Fowler type resonant boundary problem.
In Section 5 we consider an example.
2. Auxiliary Results
For instance, if the linear part is then Green's function for Dirichlet problem (15) in the interval is given by and satisfies the estimates
Definition 4 (see [14, 15]). We will say that is an -type solution of the problem (6) and (7) (resp., (14) and (7)) if for small enough the difference has exactly zeros in and , where is a solution of (6) (resp., (14)), which satisfies the initial conditions
Definition 6. Let (6) and (14), where the linear part is nonresonant with respect to the boundary conditions (7) in the interval , be equivalent in the domain in the sense that any solution of (6) with a graph in is also a solution of (14) and vice versa. Suppose that any solution of the quasilinear problem (14) and (7) satisfies the estimates We will say then that the problem (6) and (7) allows for quasilinearization with respect to a domain and a linear part .
Theorem 8 (see [14, 15]). Suppose that the problem (6) and (7) allows for quasilinearization with respect to and -nonresonant linear part , and, at the same time, it allows for quasilinearization with respect to a domain and -nonresonant linear part , where . Then the problem (6) and (7) has at least 2 solutions of different types.
Corollary 9. Suppose that the problem (6) and (7) allows for quasilinearization with respect to essentially different (in the sense of Definition 3) linear parts and domains of the form (23). Then it has at least different solutions.
3. Preliminary Results
We use quasilinearization process as follows.(1)First we modify the equation adding a linear part so that the resulting linear part is not resonant yet (2)We choose constants and and truncate right side: where (3)We check the inequalities where and are the estimates of Green's function and its derivative associated with the linear part in (27); .
Proof. Apply (26) and (27). From Theorem 2 the boundary value problem (27) and (7) is solvable and has a solution , which can be written in the integral form using Green's function.
The inequalities (30) and (31) are fulfilled. This means that For these values of and the original equation (6) and the modified equation (27) are equivalent:
It follows that is also a solution of the original problem (6) and (7).
4. Application: Emden-Fowler Type Equation
Consider the Emden-Fowler type resonant problem
Proof. Let us consider instead of (34) the equivalent one
The linear part is nonresonant with respect to the boundary conditions (35) if , where is an integer. We wish to make the right side in (39) bounded. Denote
The function is odd in for fixed . Consider it for nonnegative values of . There exists point of local extremum (it is either a point of maximum in case of or a point of minimum in case of ):
Figure 1 illustrates the case of for fixed .
We can calculate the value of the function at the point of maximum . Set Choose such that The value of is computed by solving the equation or, equivalently, that of with respect to for any fixed . Computation gives that where a constant is described in (38). Set
Let us consider the quasilinear equation where .
The modified quasilinear equation (48) is equivalent to the given equation (34) in the respective domains
The problem (48) and (35) is solvable and solutions can be written in integral form where is the Green function to the respective homogeneous problem It is given by and satisfies the estimate It follows from (50) that If inequality holds, then a solution of the quasilinear problem (48) and (35) satisfies the estimate and the original problem (34) and (35) allows for quasilinearization with respect to the domain and the linear part . It follows from Theorem 5 that if the linear part is -nonresonant, then the problem (34) and (35) has an -type solution.
Then should satisfy the inequalities or where .
Consider inequality (55) and assume that satisfies the estimates (36). If , then but in the case of we have Hence inequality (55) reduces to (37).
We have computed results (see Table 1) for , various , which show that some satisfy inequality (37). It may happen that several fall into the same nonresonance interval . In Table 1 we select only one for any respective nonresonance interval (this prevents an error when estimating the number of solutions of different types). For instance, in the case , two values of , namely, and , fall into the same nonresonance interval . We show only in Table 1. Intervals of nonresonance are given in the third column of Table 1.
Example 13. For instance, we consider the resonant equation
with boundary conditions (35).
Function is odd. And for we can rewrite
Rewrite (62) equivalently: The linear part in (63) is no more resonant with respect to (35).
We would like to make the function bounded and still continuous. The function is an odd function with a maximum at (cf. Figure 2). Define . Solve the equation for . The solution is .
Define the truncated function In this example it is
The function is continuous and bounded by the number (cf. Figure 3). Therefore the problem has a solution . Let us show that and hence is also a solution of the problem (61) and (35).
A solution of (66) satisfies the integral equation where is the Green function for the problem It follows from (67) that where , holds.
Then, satisfies, since This means that the problem (61) allows for quasilinearization with respect to the linear part , where and one can only propose that there exists 1-type solution.
The problem allows for three essentially different quasilinearizations with , and . Then there exist at least 3 solutions of different types. First, it has the trivial solution, which is a 1-type solution. Figure 4 illustrates the 2-type and 3-type solutions of the boundary value problem (72).
5. Example: Equation
We consider the boundary value problem Similar to the previous example we rewrite the equation equivalently: The function is odd and we would like to make this function bounded and still continuous. The function has a maximum at . Define . Solve the equation for . The solution is .
We define the truncated function which is bounded and continuous. The quasilinear boundary value problem has a solution . We can write it through Green's function, where the estimate of Green's function is
This means that for this example the problem (73) allows for quasilinearization with respect to the linear part , where . Using quasilinearization process we can say that resonant boundary problem (73) is solvable.
We show that the resonant boundary value problem can be studied by using a quasilinearization process. The respective cases are considered, when the differential equations of resonant type with bounded or unbounded right side function allow for quasilinearization. As an application the conditions for solvability of the Emden-Fowler type resonant boundary value problem are given. Two examples are considered in detail showing the quasilinearization approach in action. By using quasilinearization process with different linear parts we can state the existence of different solutions of a problem thus obtaining multiplicity of results.
Conflict of Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
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