Necessary and sufficient conditions for the existence of a solution of a boundary-value problem for the Schrödinger equation are obtained in the linear and nonlinear cases. Analytic solutions are represented using the generalized Green operator.
Keywords:normally resolvable operator; generalized Green operator; Schrödinger equation
The Schrödinger equation is the subject of numerous publications, and it is impossible to analyze all of them in detail. For this reason, we only briefly describe the methods and ideas that underlie the approach proposed in this paper for the investigation of the linear and a weakly nonlinear Schrödinger equation with different boundary conditions.
In this work, we develop constructive methods of analysis of linear and weakly nonlinear boundary-value problems, which occupy a central place in the qualitative theory of differential equations. The specific feature of these problems is that the operator of the linear part of the equation does not have an inverse. This does not allow one to use the traditional methods based on the principles of contracting mappings and a fixed point. These problems include the most complicated and inadequately studied problems known as critical (or resonance) problems [1-4]. Therefore, for the investigation of periodic problems for the Schrödinger equation, we develop the technique of generalized inverse operators [5-8] for the original linear operator in Banach and Hilbert spaces.
On the other hand, we use the notion of a strong generalized solution of an operator equation developed in . The origins of this approach go back to the works of Weil and Sobolev. Using the process of completion, one can introduce the concept of a strong pseudoinverse operator for an arbitrary linear bounded operator and thus relax the requirement that the range of its values be closed. In this way, one can prove the existence of solutions of different types for the linear Schrödinger equation with arbitrary inhomogeneities. Thus, one may say that, in a certain sense, the Schrödinger equation is always solvable. There are three possible types of solutions: classical generalized solutions, strong generalized solution, and strong pseudosolutions .
For the analysis of a weakly nonlinear Schrödinger equation, we develop the ideas of the Lyapunov-Schmidt method and efficient methods of perturbation theory, namely the Vishik-Lyusternik method . The combination of different approaches allows us to take a different look at the Schrödinger equation with a constant unbounded operator in the linear part and obtain all its solutions by using the generalized Green operator of this problem constructed in this work. Possible generalizations are discussed in the final part of the paper. By an example of the abstract van der Pol equation, we illustrate the results that can be obtained by using the proposed method.
Auxiliary result (linear case)
Statement of the problem
where , H is Hilbert space and vector-function is integrable; for simplicity, the unbounded operator has the following form  for each :
where T is a strongly positive self-adjoint operator in the Hilbert space H. Since the operator T is closed, the domain of the operator T is a Hilbert space with scalar product . The operator is self-adjoint in the domain with product
the infinitesimal generator of a strongly continuous evolution semigroup has the form
where . Consider the case where the set of values of is closed . Since for all , we can conclude  that the operator system (3) is solvable if and only if
1. Solutions of the boundary-value problem (1), (2) exist if and only if
2. Under condition (5), solutions of (1), (2) have the form
(1) Classical generalized solutions.
is the generalized Green operator (or it has the form of a convergent series).
is linear, continuous, and injective. Here,
are a continuous bijection and a projection, respectively. The triple is a locally trivial bundle with typical fiber . In this case [, p.26,29], we can define a strong generalized solution of the equation
We complete the space X with the norm , where . Then the extended operator
is a homeomorphism of and . By the construction of a strong generalized solution , the equation
Remark 1 It should be noted that there exist the following extensions of spaces and the corresponding operators:
(3) Strong pseudosolutions.
These elements are called strong pseudosolutions by analogy with .
We now formulate the full theorem on solvability.
Theorem 1The boundary-value problem (1), (2) is always solvable.
(a) Classical or strong generalized solutions of (1), (2) exist if and only if
(b) Under assumption (8), the solutions of (1), (2) have the form
(a) Strong pseudosolutions exist if and only if
(b) Under assumption (9), the strong pseudosolutions of (1), (2) have the form
1 Main result (nonlinear case)
1.1 Modification of the Lyapunov-Schmidt method
where q is a positive constant.
The main idea of the next results was used in  for the investigation of bounded solutions.
Let us show that this problem can be solved with the use of the following operator equation for generating amplitudes:
Theorem 2 (Necessary condition)
Suppose that the nonlinear boundary-value problem (10), (11) has a generalized solutionthat becomes one of the solutionsof the generating equation (1), (2) with constantandfor. Then this constant must satisfy the equation for generating amplitudes (12).
Proof If the boundary-value problem (10), (11) has classical generalized solutions, then, by Lemma 1, the following solvability condition must be satisfied:
By using condition (5), we establish that condition (13) is equivalent to the following:
To find a sufficient condition for the existence of solutions of the boundary-value problem (10), (11), we additionally assume that the operator function is strongly differentiable in a neighborhood of the generating solution ().
This problem can be solved with the use of the operator
Theorem 3 (Sufficient condition)
This solution can be found by using the following iterative process:
1.2 Relationship between necessary and sufficient conditions
First, we formulate the following assertion:
CorollarySuppose that a functionalhas the Fréchet derivativefor each elementof the Hilbert spaceHsatisfying the equation for generating constants (12). Ifhas a bounded inverse, then the boundary-value problem (10), (11) has a unique solution for each.
Remark 2 If the assumptions of the corollary are satisfied, then it follows from its proof that the operators and are equal. Since the operator is invertible, it follows that assumptions 1 and 2 of Theorem 3 are necessarily satisfied for the operator . In this case, the boundary-value problem (10), (11) has a unique bounded solution for each satisfying (12). Therefore, the invertibility condition for the operator expresses the relationship between the necessary and sufficient conditions. In the finite-dimensional case, the condition of invertibility of the operator is equivalent to the condition of simplicity of the root of the equation for generating amplitudes .
In this way, we modify the well-known Lyapunov-Schmidt method. It should be emphasized that Theorems 2 and 3 give us a condition for the chaotic behavior of (10) and (11) .
We now illustrate the obtained assertion. Consider the following differential equation in a separable Hilbert space H:
where T is an unbounded operator with compact . Then there exists an orthonormal basis such that and , . In this case, the operator system (10), (11) for the boundary-value problem (14), (15) is equivalent to the following countable system of ordinary differential equations ():
We find the solutions of these equations in the space that, for , turn into one of the solutions of the generating equation. Consider the critical case , . Let . In this case, the set of all periodic solutions of (16), (17) has the form
Then we can obtain the next result.
Theorem 4 (Necessary condition for the van der Pol equation)
Suppose that the boundary-value problem (16), (17) has a bounded solutionthat becomes one of the solutions of the generating equations with pairs of constants, . Then only a finite number of these pairs are not equal to zero. Moreover, if, , then these constants lie on anN-dimensional torus in the infinite-dimensional space of constants:
Remark Similarly, we can study the Schrödinger equation with a variable operator and more general boundary conditions (as noted in the introduction).
Consider the differential Schrödinger equation
in a Hilbert space H with the boundary condition
where, for each , the unbounded operator has the form , is an unbounded self-adjoint operator with domain , and the mapping is strongly continuous. The operator Q is linear and bounded and acts from the Hilbert space H to . As in , we define the operator-valued function
In this case, admits the Dyson representation [, p.311]; denote its propagator by . If , then is a weak solution of (14) with the condition in the sense that, for any , the function is differentiable and
A detailed study of the boundary-value problem (18), (19) will be given in a separate paper.
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