arXiv subjects
Publications and source records attributed to Konrad Schmüdgen.
Let $A$ and $B$ be unbounded self-adjoint operators on a Hilbert space. Suppose that there exists a dense linear subspace $D\subseteq D(AB)\cap D(BA)$ such that $ABx=BAx$ for $x \in D$. We give some characterizations of the strong commutativity of $A$ and $B$ in terms of domain equalities.
An operator $A$ on a complex Hilbert space $\Hh$ is called antilinear if $A(x+y)=Ax+Ay$ and $A(λx)=\ovλ Ax$ for $x,y\in \cD(A)$ and $λ\in \dC$. We investigate some classes of densely defined antilinear unbounded operators, especially antilinear normal operators. We give various characterizations of antilinear normal operators and study a class of such operators in detail. Our main result is a structure theorem for unbounded antilinear normal operators.
Let $C$ be a conjugation on a Hilbert space $\mathcal{H}$. A densely defined linear operator $A$ on $\mathcal{H}$ is called $C$-symmetric if $CAC\subseteq A^*$ and $C$-self-adjoint if $CAC=A^*$. Our main results describe all $C$-self-adjoint extensions of $A$ on $\mathcal{H}$. Further, we prove a $C$-self-adjointness criterion based on quasi-analytic vectors and we characterize $C$-self-adjoint operators in terms of their polar decompositions.
Given a conjugation (involution) $C$ on a Hilbert space, $C$-self-adjoint contractive extensions of a non-densely defined $C$-symmetric contraction are studied and parameterizations of all such extensions are obtained. As an application, a proof of an announced result of Glazman \cite{Glazman1} on the existence of maximal dissipative $C$-self-adjoint extensions of a densely defined $C$-symmetric dissipative operator is given.
We study $K$-positivity preservers with given closed $K\subseteq\mathbb{R}^n$, i.e., linear maps $T:\mathbb{R}[x_1,\dots,x_n]\to\mathbb{R}[x_1,\dots,x_n]$ such that $T\mathrm{Pos}(K)\subseteq\mathrm{Pos}(K)$ holds, and their generators $A:\mathbb{R}[x_1,\dots,x_n]\to\mathbb{R}[x_1,\dots,x_n]$, i.e., $e^{tA}\mathrm{Pos}(K)\subseteq\mathrm{Pos}(K)$ holds for all $t\geq 0$. We characterize these maps $T$ for any closed $K\subseteq\mathbb{R}^n$ in Theorem 4.5. We characterize the maps $A$ in Theorem 5.12 for $K=\mathrm{R}^n$ and give partial results for general $K$. In Proposition 6.1 and 6.3 we give maps $A$ such that $e^{tA}$ is a positivity preserver for all $t\geq τ$ for some $τ>0$ but not for $t\in (0,τ)$, i.e., we have an eventually positive semi-group.
T. M. Bisgaard proved that the $*$-algebra ${\bf C}[z,\overline{z},1/z\overline{z}]$ has the moment property, that is, each positive linear functional on this $*$-algebra is a moment functional. We generalize this result to polynomials in $d$ variables $z_1,...,z_d$. We prove that there exist $3d-2$ linear polynomials as denominators such that the corresponding $*$-algebra has the moment property, while for 3 linear polynomials in case $d=2$ the moment property always fails. Further, it is shown that for the real algebras ${\bf R}[x,y,1/(x^2+y^2)]$ (the hermitean part of ${\bf C}[z,\overline{z},1/z\overline{z}]$) and ${\bf R}[x,y,x^2/(x^2+y^2),xy/(x^2+y^2)]$, all positive semidefinite elements are sums of squares. These results are used to prove that for the semigroup $*$-algebras of ${\bf Z}^2$, ${\bf N}_0\times{\bf Z}$ and ${\mathsf N}_+:=\{(k,n)\in{\bf Z}^2:k+n\geq 0\}$, all positive semidefinite elements are sums of hermitean squares.
Suppose that $\sA$ is a finitely generated commutative unital real algebra and $K$ is a closed subset of the set $\hat{A}$ of characters of $\sA$. We study the following problem: When is {\it each} linear functional $L:{\sA} \to \dR$ an integral with respect to some signed Radon measure on $\hat{\sA}$ supported by the set $K$? A complete characterization of the sets $K$ and algebras $\sA$ by necessary and sufficient conditions is given. The result is applied to the polynomial algebra $\dR[x_1,\dots,x_d]$ and subsets $K$ of $\dR^d$.
A variant of the Archimedean Positivstellensatz is proved which is based on Archimedean semirings or quadratic modules of generating subalgebras. It allows one to obtain representations of strictly positive polynomials on compact semi-algebraic by means of smaller sets of squares or polynomials. A large number of examples is developed in detail.
We continue the study of truncated matrix-valued moment problems begun in arXiv:2310.00957. Let $q\in\mathbb{N}$. Suppose that $(\mathcal{X},\mathfrak{X})$ is a measurable space and $\mathcal{E}$ is a finite-dimensional vector space of measurable mappings of $\mathscr{X}$ into $\mathcal{H}_q$, the Hermitian $q\times q$ matrices. A linear functional $Λ$ on $\mathcal{E}$ is called a moment functional if there exists a positive $\mathcal{H}_q$-valued measure $μ$ on $(\mathcal{X},\mathfrak{X})$ such that $Λ(F)=\int_\mathcal{X} \langle F, \mathrm{d}μ\rangle$ for $F\in \mathcal{E}$. In this paper a number of special topics on the truncated matricial moment problem are treated. We restate a result from (Mourrain and Schmüdgen, 2016) to obtain a matricial version of the flat extension theorem. Assuming that $\mathcal{X}$ is a compact space and all elements of $ \mathcal{E}$ are continuous on $\mathcal{X}$ we characterize moment functionals in terms of positivity and obtain an ordered maximal mass representing measure for each moment functional. The set of masses of representing measures at a fixed point and some related sets are studied. The class of commutative matrix moment functionals is investigated. We generalize the apolar scalar product for homogeneous polynomials to the matrix case and apply this to the matricial truncated moment problem.
This paper is about the general truncated matrix-valued moment problem. Let $\mathcal{H}_q$ denote the complex Hermitian $q\times q$-matrices, $q\in \mathbb{N}$. Suppose that $(\mathcal{X},\mathfrak{X})$ is a measurable space and $\mathcal{E}$ is a finite-dimensional vector space of measurable mappings of $\mathcal{X}$ into $\mathcal{H}_q$. A linear functional $Λ$ on $\mathcal{E}$ is called a moment functional if there exists a positive $\mathcal{H}_q$-valued measure $μ$ on $(\mathcal{X},\mathfrak{X})$ such that $Λ(F)=\int_\mathcal{X} \langle F,\mathrm{d}μ\rangle$ for $F\in \mathcal{E}$. We prove a matricial version of the Richter-Tchakaloff theorem which states that each moment functional on $\mathcal{E}$ has a finitely atomic representing measure. It is shown that strictly positive linear functionals on $\mathcal{E}$ are moment functionals. For a moment functional $Λ$, we study the set of atoms $\mathcal{W}(Λ)$ and the Carathéodory numbers $\mathrm{Car}(Λ)$, $\mathrm{car}(Λ)$ and we define and investigate the core set $\mathcal{V}(Λ)$. A main result of the paper is the equality $\mathcal{W}(Λ)=\mathcal{V}(Λ)$.
The following is an improved version of Chapter 12 of my book [Sm17]. Among others, we present a new unified approach to the Archimedean Positivstellensätze for quadratic modules and semirings in Section 12.4 and we add a number of new results on Positivstellensätze for semirings and the corresponding moment problems.All references to formulas and to the bibliography of the book are retained. This version is essentially based on results from the recent paper [SmS23]. We will also use a result form book [Sm20].
In this paper we develop a number of results and notions concerning Positivstellensätze for semirings (preprimes) of commutative unital real algebras. First we reduce the Archimedean Positivstellensatz for semirings to the corresponding result for quadratic modules. Various applications of the Archimedean Positivstellensatz for semirings are investigated. A general Positivstellensatz with denominators is proved for filtered algebras with semirings. As an application we derive a denominator-free Positivstellensatz for the cylindrical extension of an algebra with Archimedean semiring. A large number of illustrating examples are given.
An adjoint pair is a pair of densely defined linear operators $A, B$ on a Hilbert space such that $\langle Ax,y\rangle=\langle x,By\rangle$ for $x\in \cD(A), y \in \cD(B).$ We consider adjoint pairs for which $0$ is a regular point for both operators and associate a boundary triplet to such an adjoint pair. Proper extensions of the operator $B$ are in one-to-one correspondence $T_\cC\leftrightarrow \cC $ to closed subspaces $\cC$ of $\cN(A^*)\oplus\cN(B^*)$. In the case when $B$ is formally normal and $\cD(A)=\cD(B)$, the normal operators $T_\cC$ are characterized. Next we assume that $B$ has an extension to a normal operator with bounded inverse. Then the normal operators $T_\cC$ are described and the case when $\cN(A^*)$ has dimension one is treated.
We shall say that a densely defined closed operator $T$ on a Hilbert space is balanced if $\cD(T)=\cD(T^*)$. Balanced operators are described in terms of their phase operators abnd their moduli. Examples of balanced operators are developed. A characterization of the domain equality $\cD(A)=\cD(B)$ for positive self-adjoint operators $A$ and $B$ with bounded inverses is given in terms of their spectral measures.
We prove a solvability theorem for the Stieltjes moment problem on $R^d$ which is based on the multivariate Stieltjes condition $\sum_{n=1}^\infty L(x_j^n)^{-1/(2n)}=+\infty$, $j=1,\dots,d.$ This result is applied to derive a new solvability theorem for the moment problem on unbounded semi-algebraic subsets of $R^d$.
These are ten lectures on the moment problem delivered by the author at the Vietnam Institute of Advanced Studies in Mathematics, Hanoi, March 2019. The first 3 lectures are about the one-dimensional full and truncated moment problem. The next 4 lectures deal with the multidimensional full moment problem with particular emphasis on the interactions with real algebraic geometry and polynomial optimization. The final 3 lectures are devoted to the truncated multidimensional moment problem and some recent developments in this area..
A $*$-bimodule for a unital $*$-algebra $A$ is an $A$-bimodule $X$ which is a vector space with involution $x\mapsto x^+$ satisfying $(a\cdot x\cdot b)^+=b^+\cdot x^+\cdot b^+$ for $x\in X$ and $a,b\in A$. An algebraic model for $*$-bimodules is given. Hilbert space representations of $*$-bimodules are defined and studied. A GNS-like representation theorem is obtained.
Let $\mathsf{A}=\{a_1,\dots,a_m\}$, $m\in\mathbb{N}$, be measurable functions on a measurable space $(\mathcal{X},\mathfrak{A})$. If $μ$ is a positive measure on $(\mathcal{X},\mathfrak{A})$ such that $\int a_i dμ<\infty$ for all $i$, then the sequence $(\int a_1 dμ,\dots,\int a_m dμ)$ is called a moment sequence. By Richter's Theorem each moment sequence has a $k$-atomic representing measure with $k\leq m$. The set $\mathcal{S}_\mathsf{A}$ of all moment sequences is the moment cone. The aim of this paper is to analyze the various structures of the moment cone. The main results concern the facial structure (exposed faces, facial dimensions) and lower and upper bounds of the Carathéodory number (that is, the smallest number of atoms which suffices for all moment sequences) of the convex cone $\mathcal{S}_{\mathcal{A}}$. In the case when $\mathcal{X}\subseteq \mathbb{R}^n$ and $a_i\in C^1(\mathcal{X},\mathbb{R})$, the differential structure of the moment map and regularity/singularity properties of moment sequences are analyzed. The maximal mass problem is considered and some applications to other problems are sketched.