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Yicao Wang

Publications and source records attributed to Yicao Wang.

10 recordsLinked to original sources

Self-adjoint extensions with compact resolvent

Let $T$ be a densely defined closed symmetric operator with equal deficiency indices in a separable complex Hilbert space $H$. In this paper, we prove that $T$ has a self-adjoint extension with compact resolvent if and only if the domain $D(T)$ of $T$ is compactly embedded in $H$ w.r.t. the graph norm on $D(T)$. If it is the case, we also prove that all self-adjoint extensions with compact resolvent can be parameterized by unitary operators $U$ on a certain Hilbert space such that $U-Id$ is compact.

math.FA

Complex analysis of symmetric operators. II: entire operators with deficiency index 1

This paper is a continuation of our previous work \cite{wang2024complex}. It mainly deals with entire operators $T$ with deficiency index 1 \emph{systematically} from the complex-geometric viewpoint proposed in \cite{wang2024complex}. We pay special attention to the characteristic line bundle $F$ of $T$. We investigate its curvature in detail and demonstrate how it is connected to the height function of $T$ and to the distribution of zeros of elements in the canonical model Hilbert space which consists of certain holomorphic sections of $F$. This study is applied to an indeterminate Hamburger moment problem to show the growth property of the associated Jacobi operator coincides with that defined in terms of entries of the Nevanlinna matrix. We also show how various functional models for $T$ can be derived from our canonical model by restricting $F$ to certain subsets of $\mathbb{C}$ and choosing suitable trivializations. This makes the interrelationships among these models much more transparent. By introducing the mean type of a generic non-self-adjoint extension and using the de Branges-Rovnyak model, we show the mean type is the only obstruction to completeness of such an extension. We also prove that the measure of incomplete extensions is zero. Some other new results and new proofs of old results are also included.

math.FA

Complex analysis of symmetric operators, I

Based on the relationship of symmetric operators with Hermitian symmetric spaces, we introduce the notion of \emph{Weyl curve} for a symmetric operator $T$, which is the geometric abstraction and generalization of the well-known Weyl functions. We prove that there is a one-to-one correspondence between unitary equivalence classes of simple symmetric operators and congruence classes of Nevanlinna curves (the geometric analogue of operator-valued Nevanlinna functions). To prove this result, we introduce a \emph{canonical} functional model for $T$ in terms of its \emph{characteristic vector bundles}. In this geometric formalism, when the deficiency indices are $(n,n)$ ($n\leq +\infty$) we also introduce the notion of \emph{entire operators}, whose Weyl curves are entire in the Grassmannian $Gr(n,2n)$. If $n$ is finite, this makes it possible to introduce modern value distribution theory of entire curves into the picture and to demonstrate that the distribution of eigenvalues of a \emph{generic} abstract boundary value problem is the same thing as the value distribution of the Weyl curve with respect to the Cartier divisor induced by the corresponding boundary condition. Many other new concepts are introduced and many new results are obtained as well.

math.FA

Toric generalized Kaehler structures. III

The paper clarifies some subtle points surrounding the definition of scalar curvature in generalized K$\ddot{a}$hler (GK) geometry. We have solved an open problem in GK geometry of symplectic type posed by R. Goto \cite{Go1} on relating the scalar curvature defined in terms of generalized pure spinors \emph{directly} to the underlying biHermitian structure. In particular, we apply this solution to toric GK geometry of symplectic type and prove that the scalar curvature suggested in this setting by L. Boulanger \cite{Bou} coincides with Goto's definition.

math.SG

Toric generalized Kaehler structures

Anti-diagonal toric generalized K$\ddot{a}$hler structures of symplectic type on a compact toric symplectic manifold were investigated in \cite{Wang2} . In this article, we consider \emph{general} toric generalized K$\ddot{a}$hler structures of symplectic type, without requiring them to be anti-diagonal. Such a structure is characterized by a triple $(τ, C, F)$ where $τ$ is a strictly convex function defined in the interior of the moment polytope $Δ$ and $C, F$ are two constant anti-symmetric matrices. We prove that underlying each such a structure is a \emph{canonical} toric K$\ddot{a}$hler structure $I_0$ whose symplectic potential is given by this $τ$, and when $C=0$ the generalized complex structure $\mathbb{J}_1$ other than the symplectic one arises from an $I_0$-holomorphic Poisson structure $β$ in a \emph{novel} way not mentioned in the literature before. Conversely, given a toric K$\ddot{a}$hler structure with symplectic potential $τ$ and two anti-symmetric constant matrices $C, F$, the triple $(τ, C, F)$ then determines a toric generalized K$\ddot{a}$hler structure of symplectic type canonically if $F$ satisfies additionally a certain positive-definiteness condition. In particular, if the initial toric K$\ddot{a}$hler manifold is the standard $M_Δ$ associated to a Delzant polytope $Δ$, the resulting generalized K$\ddot{a}$hler structure can be interpreted as obtained via generalized K$\ddot{a}$hler reduction from a generalized K$\ddot{a}$hler structure on an open subset of a complex linear space, just as in Delzant's construction $M_Δ$ is obtained through K$\ddot{a}$hler reduction from a complex linear space.

math.DG

Toric generalized K$\ddot{a}$hler structures. I

This is a sequel of \cite{Wang}, which provides a general formalism for this paper. We mainly investigate thoroughly a subclass of toric generalized K$\ddot{a}$hler manifolds of symplectic type introduced by Boulanger in \cite{Bou}. We find torus actions on such manifolds are all \emph{strong Hamiltonian} in the sense of \cite{Wang}. For each such a manifold, we prove that besides the ordinary two complex structures $J_\pm$ associated to the biHermitian description, there is a \emph{third} canonical complex structure $J_0$ underlying the geometry, which makes the manifold toric K$\ddot{a}$hler. We find the other generalized complex structure besides the symplectic one is always a B-transform of a generalized complex structure induced from a $J_0$-holomorphic Poisson structure $β$ characterized by an anti-symmetric constant matrix. Stimulated by the above results, we introduce a \emph{generalized Delzant construction} which starts from a Delzant polytope with $d$ faces of codimension 1, the standard K$\ddot{a}$hler structure of $\mathbb{C}^d$ and an anti-symmetric $d\times d$ matrix. This construction is used to produce non-abelian examples of strong Hamiltonian actions.

math.DG

Metric Reduction and Generalized Holomorphic Structures

In this paper, metric reduction in generalized geometry is investigated. We show how the Bismut connections on the quotient manifold are obtained from those on the original manifold. The result facilitates the analysis of generalized K$\ddot{a}$hler reduction, which motivates the concept of metric generalized principal bundles and our approach to construct a family of generalized holomorphic line bundles over $\mathbb{C}P^2$ equipped with some non-trivial generalized K$\ddot{a}$hler structures.

math.DG

The GIT aspect of generalized K$\ddot{a}$hler reduction. I

We revisit generalized K$\ddot{a}$hler reduction introduced by Lin and Tolman in \cite{LT} from a viewpoint of geometric invariant theory. It is shown that in the strong Hamiltonian case introduced in the present paper, many well-known conclusions of ordinary K$\ddot{a}$hler reduction can be generalized without much effort to the generalized setting. It is also shown how generalized holomorphic structures arise naturally from the reduction procedure.

math.DG

Metric Reduction in Generalized Geometry and Balanced Topological Field Theories

The recently established metric reduction in generalized geometry is encoded in 0-dimensional supersymmetric $σ$-models. This is an example of balanced topological field theories. To find the geometric content of such models, the reduction of Bismut connections is studies in detail. Generalized K$\ddot{a}$hler reduction is briefly revisited in this formalism and the generalized K$\ddot{a}$hler geometry on the moduli space of instantons on a generalized K$\ddot{a}$hler 4-manifold of even type is thus explained formally in a topological field theoretic way.

math-ph

Geometric aspects of self-adjoint Sturm-Liouville problems

In the paper, we use $\mathrm{U}(2)$, the group of $2\times 2$ unitary matices, to parameterize the space of all self-adjoint boundary conditions for a fixed Sturm-Liouville equation on the interval $[0,1]$. The adjoint action of $\mathrm{U}(2)$ on itself naturally leads to a refined classification of self-adjoint boundary conditions--each adjoint orbit is a subclass of these boundary conditions. We give explicit parameterizations of those adjoint orbits of principal type, i.e. orbits diffeomorphic to the 2-sphere $S^2$, and investigate the behavior of the $n$-th eigenvalue $λ_n$ as a function on such orbits.

math-ph