Abstract:
Schrödinger's equation with potential that is a sum of a regular function and a finite set of point scatterers of Bethe–Peierls type is under consideration. For this equation the spectral problem with homogeneous linear boundary conditions is considered, which covers the Dirichlet, Neumann, and Robin cases. It is shown that when the energy E is an eigenvalue with multiplicity m, it remains an eigenvalue with multiplicity at least m−n after adding n<m point scatterers. As a consequence, because for the zero potential all values of the energy are transmission eigenvalues with infinite multiplicity, this property also holds for n-point potentials, as discovered originally in a recent paper by the authors.
Bibliography: 33 titles.
This research was carried out with the support of a joint grant of the Russian Foundation for Basic Research
and the Centre National de la Recherche Scientifique (RFBR project 20-51-15004/PRC no. 2795 CNRS/RFBR).
To Iskander Asanovich Taimanov on his 60th birthday
1. Introduction
The important role of exactly solvable models in quantum mechanics is well known. For equations of Schrödinger type there are many important potentials for which, in dimension d=1 this equation is exactly solvable for all energies, while in dimension d=2 it is exactly solvable for a single particular value of the energy. In the construction and investigation of such potentials methods of soliton theory are efficient, for instance, transformations of Darboux–Moutard type. The literature devoted to this line of research is quite extended. Without attempting to present a complete list, we note, for example, [1]–[8] and the references in these works. On the other hand, in dimension d=2 and higher, apart from equations with highly symmetric potential, the stock of cases that are exactly solvable for all energies is quite limited. One such case is Schrödinger’s equation with potential equal to a sum of point potentials of Bethe–Peierls type. For the first time such a point potential was introduced in [9] for d=3, to describe the interaction between neutrons and protons. Subsequently, point and multipoint scatterers were considered by many authors, including Thomas [10], Fermi [11], Zel’dovich [12], Berezin and Faddeev [13]; also see the monograph [14]. Also nowadays such potentials are under extensive investigation (for instance, see [3] and [15]–[25], as well as the references there).
In this paper we consider the stationary Schrödinger equation (in dimensionless variables)
−Δψ+V(x)ψ=Eψ,V(x)=v0(x)+v(x),x∈Rd,d=1,2,3,
where v0(x) is a sufficiently regular real function on Rd which increases at infinity sufficiently rapidly, and v(x) is a sum of n point scatterers which can formally be expressed as
v(x)=n∑j=1εjδ(x−yj).
It is well known that point scatterers can only be defined in dimensions d=1,2,3 (for instance, see [3]). If d=1, then a point scatterer is just a standard Dirac delta function with an arbitrary coefficient. For d=2 or d=3, εδ(x) denotes a ‘renormalized’ delta function depending on the real parameter ε=ε(α).
More precisely, a complex function ψ is a solution of (1) if the following conditions are satisfied:
1) away from the points yj the function ψ satisfies Schrödinger’s equation (1) with potential v0(x);
2) the following limit relations hold in a neighbourhood of the points yj:
In (2)–(7) we use the parameter αj which codes the strength of the jth scatterer (see [3] and [22]). Recall that the standard physical approach to the derivation of relations (5) and (7) for the wave function ψ is to consider a potential well v(x) of radius r and depth U(r), after which r is let tend to 0, while the function U(r) is chosen so that a non-trivial limiting solution ψ exists. In this case the ‘renormalization’ of a delta function means that U(r)=o(r−d) for d=2,3, so that from the standpoint of distributions the potential v(x) tends to zero, rather than to an ordinary Dirac delta function. Using another approach, based on the momentum representation, a point potential for d=3 was mathematically defined and investigated in [13].
Remark 1. In the definition of a point scatterer we can start from a potential well of any shape and make scaling; for a generic well the limit does not depend on its initial shape. However, we can select a special shape of the well so as to construct another point potential, which can be regarded as a ‘renormalized’ δ′-function (for d=3, see [26]).
Remark 2. One motivation for introducing point potentials for d=3 in [9] was the absence of any kind of complete theory of strong interactions at that time. However, the simple idea that if the object has size much less than the wavelength, then we can treat it as a point, turned out to be efficient. The use of point potentials made it possible in [11], in particular, to explain why slow neutrons interact stronger with nuclei than fast ones [27]. The reason is that in wave problems, when the length of the scattered wave (for instance, the de Broglie wave of the scattered particle in quantum mechanics or the acoustic wave in acoustics problems) is much larger than the geometric size of the scatterer, then the effective scattering cross-section can be much larger than the geometric cross-section of the scatterer: it can have the order of the square of the wavelength. On the other hand, if the size of the scatterer is much larger than the wavelength, then the scattering cross-section is determined just by the size of the scatterer.
In this paper we continue to develop the spectral theory of equation (1). More precisely, we consider a homogeneous boundary condition for this equation, including the Dirichlet, Neumann, and Robin cases, and the case of transmission eigenvalues. For such spectral problems we show, in particular, that if the energy E is an eigenvalue with multiplicity m>n for equation (1), where V(x)≡v0(x), then, in the case when V(x)=v0(x)+v(x), where v(x) is the n-point potential defined above, this energy is also an eigenvalue with multiplicity at least m−n.
This paper is a continuation of [22]. In particular, for d=2,3, when v0(x)≡0 all energies are transmission eigenvalues with infinite multiplicity. Hence this property also holds for all n-point potentials, as discovered in [22].
2. Dirichlet, Neumann, and Robin problems
Consider equation (1) with Robin boundary condition
a(x)ψ(x)+b(x)∂ψ(x)∂ν|∂D=0
in a bounded domain D with regular boundary ∂D, where ∂/∂ν is the derivative in the direction of the outward normal ν to the boundary of the domain. Condition (8) covers the Dirichlet and Neumann boundary conditions as special cases. We also assume that suppv(x) lies in the interior of D.
Theorem 1. Let E be an eigenvalue with multiplicity m>n for problem (1), (8), where V(x)≡v0(x). Then E is also an eigenvalue with multiplicity at least m−n for problem (1), (8) where V(x)=v0(x)+v(x) and v(x) is the potential from (2).
Remark 3. To our knowledge the result of Theorem 1 is new even for v0≡0.
Remark 4. A nice illustration to Theorem 1 is the case when D is a ball in R3, v0≡0, and the Robin condition reduces to the Diriclet or Neumann one. The reason is that there are eigenvalues with an arbitrarily high multiplicity in this problem.
Lemma 1. Let ψ(x) satisfy equation (1) for V(x)≡v0(x). Also let ψ(yj)=0, j=1,…,n, where the points yj are as in (2). Then ψ(x) also satisfies equation (1) for V(x)=v0(x)+v(x).
Lemma 1 follows from the definition of a solution of (1) for V(x)=v0(x)+v(x), which involves formulae (3)–(7).
Proof of Theorem 1. Let ψl(x), l=1,…,m, be linearly independent eigenfunctions of spectral problem (1), (8) for V(x)≡v0(x). Consider the system of n linear equations with respect to the m unknowns zl:
m∑l=1ψl(yj)zl=0,j=1,…,n.
Its space of solutions is at least (m−n)-dimensional. Now, for each set of zl satisfying (9) the function
Ψ(x)=m∑l=1zlψl(x)
solves (1) for V(x)≡v0(x), satisfies the boundary condition (8), and vanishes at the points yj. Hence, by Lemma 1 the function Ψ(x) satisfies (1) for V(x)≡v0(x)+v(x) and satisfies the boundary condition (8). Moreover, as the solutions ψl(x) are linearly independent, Ψ(x) is distinct from identical zero if at least one coefficient zl is distinct from zero. Hence the space of functions Ψ(x) constructed in accordance with (9) and (10) has dimension at least m−n. □
3. Transmission eigenvalues
The problem of transmission eigenvalues is currently a subject of significant interest: see, for instance, [28]–[33] and [22].
An energy E is called an interior transmission eigenvalue for equation (1) in a domain D if there exists a non-trivial pair of functions ϕ(x), ψ(x) such that
ψ(x)satisfies (1) in D,
−Δϕ(x)=Eϕ(x),x∈D,
and
ψ(x)≡ϕ(x),∂∂νψ(x)≡∂∂νϕ(x)for allx∈∂D.
The dimension of the space of such pairs is called the multiplicity of the interior transmission eigenvalue. Note that for V(x)≡0 and d>1 this multiplicity is infinite.
As in § 2, we assume that suppv(x) lies in the interior of D.
Theorem 2. Let E be an interior transmission eigenvalue with multiplicity m>n in the sense of (11)–(13) for equation (1) in the domain D, where V(x)≡v0(x). Then E is also an interior transmission eigenvalue with multiplicity at least m−n in the sense of (11)–(13) for problem (1), where V(x)=v0(x)+v(x) and v(x) is the potential in (2).
The proof of Theorem 2 is similar to the proof of Theorem 1.
There are many papers containing results of the type: “if the potential V(x) is a sufficiently regular function in D, then the interior transmission eigenvalues are discrete and have finite multiplicities” (for instance, see [31]–[33]). On the other hand Theorem 2 for v0(x)≡0 yields our recent result in [22] that in dimension d=2,3, when the potential V(x) reduces to a multipoint scatterer v(x) of the form (2) all energies E∈C are transmission eigenvalues with infinite multiplicity.
Finally, in the context of the scattering problem for equation (1) in Rd, an energy E is called a transmission eigenvalue if 1 is an eigenvalue of the scattering operator ˆSE for (1). This can also be formulated as follows: there exists a function u(θ)∈L2(Sd−1), where Sd−1 is the unit sphere in Rd, such that (1) has a solution Ψ(x) with asymptotic behaviour
Ψ(x)=∫Sd−1ei|k|θxu(θ)dθ+o(1|x|(d−1)/2)as|x|→∞.
The dimension of the linear space of such functions u(θ) is called the multiplicity of the transmission eigenvalue. Note that for V(x)≡0 and d>1 all real positive energies are transmission eigenvalues with infinite multiplicity.
Similarly to transmission eigenvalues in a bounded domain D, many authors have published results of the form “if the potential V(x) is a sufficiently regular function with compact support on Rd, then the transmission eigenvalues are discrete and have finite multiplicities” (for instance, see [31]–[33]). On the other hand, in [5] we constructed real spherically symmetric potentials v0(x) in the Schwartz class on R2 such that for any fixed energy E=E0>0 the corresponding scattering operator ˆSE is the identity operator. As a consequence, for these potentials E0 is a transmission eigenvalue with infinite multiplicity. In addition, the following analogue of Theorem 2 holds.
Theorem 3. Let E be a transmission eigenvalue with multiplicity m>n for equation (1) in Rd, where V(x)≡v0(x). Then E is also a transmission eigenvalue for equation (1) for V(x)=v0(x)+v(x) in Rd with multiplicity at least m−n, where v(x) is the potential in (2).
The proof of Theorem 3 is similar to the proofs of Theorems 1 and 2.
As a consequence, after adding any finite set of point scatterers to the potentials v0(x) from [5] mentioned above, the energy E0 remains a transmission eigenvalue with infinite multiplicity.
In conclusion we mention that the fact that the boundary conditions are linear and homogeneous, rather than the particular form of these conditions, is significant in Theorems 1–3.
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Citation:
P. G. Grinevich, R. G. Novikov, “Spectral inequality for Schrödinger's equation with multipoint potential”, Russian Math. Surveys, 77:6 (2022), 1021–1028
This publication is cited in the following 2 articles:
M. M. Malamud, V. V. Marchenko, “On Kernels of Invariant Schrödinger Operators with Point Interactions. Grinevich–Novikov Conjecture”, Dokl. Math., 109:2 (2024), 125
M. M. Malamud, V. V. Marchenko, “On kernels of invariant Schrödinger operators with point interactions. Grinevich–Novikov problem”, Doklady Rossijskoj akademii nauk. Matematika, informatika, processy upravleniâ, 516 (2024), 31