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The Krein-von Neumann Extension and its Connection to an Abstract Buckling Problem

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 Added by Gerald Teschl
 Publication date 2009
  fields
and research's language is English




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We prove the unitary equivalence of the inverse of the Krein--von Neumann extension (on the orthogonal complement of its kernel) of a densely defined, closed, strictly positive operator, $Sgeq epsilon I_{mathcal{H}}$ for some $epsilon >0$ in a Hilbert space $mathcal{H}$ to an abstract buckling problem operator. In the concrete case where $S=bar{-Delta|_{C_0^infty(Omega)}}$ in $L^2(Omega; d^n x)$ for $Omegasubsetmathbb{R}^n$ an open, bounded (and sufficiently regular) domain, this recovers, as a particular case of a general result due to G. Grubb, that the eigenvalue problem for the Krein Laplacian $S_K$ (i.e., the Krein--von Neumann extension of $S$), [ S_K v = lambda v, quad lambda eq 0, ] is in one-to-one correspondence with the problem of {em the buckling of a clamped plate}, [ (-Delta)^2u=lambda (-Delta) u text{in} Omega, quad lambda eq 0, quad uin H_0^2(Omega), ] where $u$ and $v$ are related via the pair of formulas [ u = S_F^{-1} (-Delta) v, quad v = lambda^{-1}(-Delta) u, ] with $S_F$ the Friedrichs extension of $S$. This establishes the Krein extension as a natural object in elasticity theory (in analogy to the Friedrichs extension, which found natural applications in quantum mechanics, elasticity, etc.).



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In the first (and abstract) part of this survey we prove the unitary equivalence of the inverse of the Krein--von Neumann extension (on the orthogonal complement of its kernel) of a densely defined, closed, strictly positive operator, $Sgeq varepsilon I_{mathcal{H}}$ for some $varepsilon >0$ in a Hilbert space $mathcal{H}$ to an abstract buckling problem operator. This establishes the Krein extension as a natural object in elasticity theory (in analogy to the Friedrichs extension, which found natural applications in quantum mechanics, elasticity, etc.). In the second, and principal part of this survey, we study spectral properties for $H_{K,Omega}$, the Krein--von Neumann extension of the perturbed Laplacian $-Delta+V$ (in short, the perturbed Krein Laplacian) defined on $C^infty_0(Omega)$, where $V$ is measurable, bounded and nonnegative, in a bounded open set $Omegasubsetmathbb{R}^n$ belonging to a class of nonsmooth domains which contains all convex domains, along with all domains of class $C^{1,r}$, $r>1/2$.
We revisit the Krein-von Neumann extension in the case where the underlying symmetric operator is strictly positive and apply this to derive the explicit form of the Krein-von Neumann extension for singular, general (i.e., three-coefficient) Sturm-Liouville operators on arbitrary intervals. In particular, the boundary conditions for the Krein-von Neumann extension of the strictly positive minimal Sturm-Liouville operator are explicitly expressed in terms of generalized boundary values adapted to the (possible) singularity structure of the coefficients near an interval endpoint.
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