arXiv subjects

Aurelian Gheondea

Publications and source records attributed to Aurelian Gheondea.

At least 19 recordsLinked to original sources

arXiv2026

Localised Operator Valued Kernels Invariant under Actions of $*$-Semigroupoids

We consider positive semidefinite kernels which have values given by bounded linear operators on certain bundles of Hilbert spaces and which are invariant under actions of $*$-semigroupoids. For these kernels, we prove that there exist generalised $*$-representations of the $*$-semigroupoids on the underlying reproducing kernel Hilbert spaces or, equivalently, on the underlying minimal linearisations, we characterise when the $*$-representations are performed by means of bounded operators and show that this always happens for inverse semigroupoids. Then, we consider Hermitian kernels which have values given by bounded linear operators on certain bundles of Hilbert spaces and which are invariant under actions of $*$-semigroupoids. Only those Hermitian kernels having certain boundedness properties can produce reproducing kernel Krein spaces but uniqueness is more complicated. However, for these kernels, generalised $*$-representations can be obtained. If $*$-representations with bounded linear operators are requested, then stronger boundedness conditions on the kernels are needed.

math.FA↗

arXiv2026

Functional Models of Abelian Locally von Neumann Algebras and Direct Integrals of Locally Hilbert Spaces

We obtain a functional model for an arbitrary Abelian locally von Neumann algebra acting on a representing locally Hilbert space under the assumption that the index directed set is countable, in terms of locally essentially bounded functions on strictly inductive systems of measure spaces, which can be viewed as the reduction theory of this kind of operator algebras. Then, we single out the concept of a direct integral of locally Hilbert spaces and the concepts of locally decomposable and locally diagonlisable operators and we show that these form locally von Neumann algebras that are commutant one to each other. Finally, we show that any Abelian locally von Neumann algebra, which acts on separable representing locally Hilbert spaces and such that the index set is a sequentially finite directed set, is spatially isomorphic with the Abelian locally von Neumann algebra of all locally diagonlisable operators on a certain direct integral of locally Hilbert spaces with respect to a certain strictly inductive system of locally finite measure spaces on standard Borel spaces.

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arXiv2026

Forcing confidence: a Process Tracing approach with a dynamical systems model

We propose a continuous time dynamical system model for tracing the evolution of confidence in a small decision making group by consensus with the possibility that a forcing factor is exerted onto the confidence of the participants. We experimentally check whether a forcing factor appears, where it begins and ends and how it affects the evolution. We find that the equilibrium value of the confidence is lower in the model with the forcing factor than without it, and that the forcing factor can be identified and induces additional oscillations of the confidence level. This is probably one of the first times when a mathematical model is able to speak about visible effects on the confidence process tracing under alterations of its levels. Pragmatically we find a model that captures influence of participants' confidence level by observing oscillations and equilibrium and we experimentally test it with measures of individual "confidence that the decision is correct" throughout the group decision-making.

math.DS↗

arXiv2026

An Operator Theoretical Approach to Mercer's Theorem

This is a survey article on Mercer's Theorem in its most general form and its relations with the theory of reproducing kernel Hilbert spaces and the spectral theory of compact operators. We provide a modern introduction to the basics of the theory of reproducing kernel Hilbert spaces, an overview on Weyl's kernel and the Gaussian kernels, and finally an approach to Mercer's Theorem within the theory of reproducing kernel Hilbert spaces and the spectral theory of integral operators. This approach is reverse to the known approaches to Mercer's Theorem and sheds some light on the intricate relations between different domains in analysis.

math.FA↗

arXiv2025

Representing Locally Hilbert Spaces and Functional Models for Locally Normal Operators

The aim of this article is to explore in all remaining aspects the spectral theory of locally normal operators. In a previous article we proved the spectral theorem in terms of locally spectral measures. Here we prove the spectral theorem in terms of projective limits of certain multiplication operators with functions which are locally of type $L^\infty$. In order to do this, we first investigate strictly inductive systems of measure spaces and point out the concept of representing locally Hilbert space for which we obtain a functional model as a strictly inductive limit of $L^2$ type spaces. Then, we first obtain a functional model for locally normal operators on representing locally Hilbert spaces combined with a spectral multiplicity model on a pseudo-concrete functional model for the underlying locally Hilbert space, under a certain technical condition on the directed set. Finally, under the same technical condition on the directed set, we derive the spectral theorem for locally normal operators in terms of projective limits of certain multiplication operators with functions which are locally of type $L^\infty$ in two forms. As a consequence of the main result we sketch the direct integral representation of locally normal operators under the same technical assumptions and the separability of the locally Hilbert space. Examples of strictly inductive systems of measure spaces involving the Hata tree-like self-similar set, which justify the technical condition on a relevant case and which may open a connection with analysis on fractal sets, are included.

math.FA↗

arXiv2024

Localisation of Regularised and Multiview Support Vector Machine Learning

We prove a few representer theorems for a localised version of the regularised and multiview support vector machine learning problem introduced by H.Q. Minh, L. Bazzani, and V. Murino, Journal of Machine Learning Research, 17(2016) 1-72, that involves operator valued positive semidefinite kernels and their reproducing kernel Hilbert spaces. The results concern general cases when convex or nonconvex loss functions and finite or infinite dimensional input spaces are considered. We show that the general framework allows infinite dimensional input spaces and nonconvex loss functions for some special cases, in particular in case the loss functions are Gateaux differentiable. Detailed calculations are provided for the exponential least squares loss functions that lead to systems of partially nonlinear equations for which a particular different types of Newton's approximation methods based on the interior point method can be used. Some numerical experiments are performed on a toy model that illustrate the tractability of the methods that we propose.

math.FA↗

arXiv2024

Probability error bounds for approximation of functions in reproducing kernel Hilbert spaces

We find probability error bounds for approximations of functions $f$ in a separable reproducing kernel Hilbert space $\mathcal{H}$ with reproducing kernel $K$ on a base space $X$, firstly in terms of finite linear combinations of functions of type $K_{x_i}$ and then in terms of the projection $π^n_x$ on $\mathrm{Span}\{K_{x_i}\}^n_{i=1}$, for random sequences of points $x=(x_i)_i$ in $X$. Given a probability measure $P$, letting $P_K$ be the measure defined by $\mathrm{d} P_K(x)=K(x,x)\mathrm{d} P(x)$, $x\in X$, our approach is based on the nonexpansive operator \[L^2(X;P_K)\niλ\mapsto L_{P,K}λ:=\int_X λ(x)K_x\mathrm{d} P(x)\in \mathcal{H},\] where the integral exists in the Bochner sense. Using this operator, we then define a new reproducing kernel Hilbert space, denoted by $\mathcal{H}_P$, that is the operator range of $L_{P,K}$. Our main result establishes bounds, in terms of the operator $L_{P,K}$, on the probability that the Hilbert space distance between an arbitrary function $f\in\mathcal{H}$ and linear combinations of functions of type $K_{x_i}$, for $(x_i)_i$ sampled independently from $P$, falls below a given threshold. For sequences of points $(x_i)_{i=1}^\infty$ constituting a so-called uniqueness set, the orthogonal projections $π^n_x$ to $\mathrm{Span}\{K_{x_i}\}^n_{i=1}$ converge in the strong operator topology to the identity operator. We prove that, under the assumption that $\mathcal{H}_P$ is dense in $\mathcal{H}$, any sequence of iid samples from $P$ yields a uniqueness set with probability $1$. This result improves on previous error bounds in weaker norms, such as uniform or $L^p$ norms, which yield only convergence in probability and not a.c. convergence. Two examples that show the applicability of this result to a uniform distribution on a compact interval and to the Hardy space $H^2(\mathbb{D})$ are presented as well.

math.NA↗

arXiv2023

Partially Positive Semidefinite Maps on $*$-Semigroupoids and Linearisations

Motivated by Cuntz-Krieger-Toeplitz systems associated to undirected graphs and representations of groupoids, we obtain a generalisation of the Sz-Nagy's Dilation Theorem for operator valued partially positive semidefinite maps on $*$-semigroupoids with unit, with varying degrees of aggregation, firstly by $*$-representations with unbounded operators and then we characterise the existence of the corresponding $*$-representations by bounded operators. By linearisation of these constructions, we obtain similar results for operator valued partially positive semidefinite maps on $*$-algebroids with unit and then, for the special case of $B^*$-algebroids with unit, we obtain a generalisation of the Stinespring's Dilation Theorem. As an application of the generalisation of the Stinespring's Dilation Theorem, we show that some natural questions on $C^*$-algebroids are equivalent.

math.OA↗

arXiv2019

A Comparison of Two Generalisations of Triplets of Hilbert Spaces

We compare the concept of triplet of closely embedded Hilbert spaces with that of generalised triplet of Hilbert spaces in the sense of Berezanskii by showing when they coincide, when they are different, and when starting from one of them one can naturally produce the other one that essentially or fully coincides.

math.FA↗

arXiv2018

Notes on (the Birmak-Krein-Vishik theory on) selfadjoint extensions of semibounded symmetric operators

We give an explicit and versatile parametrization of all positive selfadjoint extensions of a densely defined, closed, positive operator. In addition, we identify the Friedrichs extension by specifying the parameter to which it corresponds. This is a manuscript that was circulated as the first part of the preprint "Two papers on selfadjoint extensions of symmetric semibounded operators", INCREST Preprint Series, July 1981, Bucharest, Romania, but never published. In this LaTeX typeset version, only typos and a few inappropriate formulations have been corrected, with respect to the original manuscript. I decided to post it on arXiv since, taking into account recent articles, the results are still of current interest. Tiberiu Constantinescu died in 2005.

math.FA↗

arXiv2017

Invariant Weakly Positive Semidefinite Kernels with Values in Topologically Ordered $*$-Spaces

We consider weakly positive semidefinite kernels valued in ordered $*$-spaces with or without certain topological properties, and investigate their linearisations (Kolmogorov decompositions) as well as their reproducing kernel spaces. The spaces of realisations are of VE (Vector Euclidean) or VH (Vector Hilbert) type, more precisely, vector spaces that possess gramians (vector valued inner products). The main results refer to the case when the kernels are invariant under certain actions of $*$-semigroups and show under which conditions $*$-representations on VE-spaces, or VH-spaces in the topological case, can be obtained. Finally we show that these results unify most of dilation type results for invariant positive semidefinite kernels with operator values as well as recent results on positive semidefinite maps on $*$-semigroups with values operators from a locally bounded topological vector space to its conjugate $Z$-dual space, for $Z$ an ordered $*$-space.

math.FA↗

arXiv2016

Operator Models for Hilbert Locally $C^*$-Modules

We single out the concept of concrete Hilbert module over a locally $C^*$-algebra by means of locally bounded operators on certain strictly inductive limits of Hilbert spaces. Using this concept, we construct an operator model for all Hilbert locally $C^*$-modules and, as an application, we obtain a direct construction of the exterior tensor product of Hilbert locally $C^*$-modules. These are obtained as consequences of a general dilation theorem for positive semidefinite kernels invariant under an action of a $*$-semigroup with values locally bounded operators. As a by-product, we obtain two Stinespring type theorems for completely positive maps on locally $C^*$-algebras and with values locally bounded operators.

math.OA↗

arXiv2016

On Propagation of Fixed Points of Quantum Operations and Beyond

We show that some abstract results on propagation of fixed points for completely positive maps on $C^*$-algebras provide a natural approach to unify recent Noether type theorems on the equivalence of symmetries with conservation laws for dynamical systems of Markov processes, of quantum operations, and of quantum stochastic maps. In addition, we obtain some new Noether type theorems, provide examples and counter-examples, and extend most of the existing results with characterisations in terms of dual infinitesimal generators of the corresponding strongly continuous one-parameter semigroups.

math.OA↗

arXiv2016

On the Dynamics of a Third Order Newton's Approximation Method

We show that the third order approximation function $M_f$, proposed by S. Amat, S. Busquier, S. Plaza, in \textit{J. Math. Anal. Appl.}, 366(2010), 24--32, for functions $f$ twice continuously differentiable and such that both $f$ and its derivative do not have multiple roots, with at least four roots, and infinite limits of opposite signs at $\pm\infty$, have periodic points of any prime period and that the set of points $a$ at which the approximation sequence $(M_f^n(a))_{n\in\mathbb{N}}$ does not converge is uncountable. In addition, we observe that in their Scaling Theorem analyticity can be replaced with differentiability.

math.DS↗

arXiv2015

Representations of *-semigroups associated to invariant kernels with values continuously adjointable operators

We consider positive semidefinite kernels valued in the $*$-algebra of continuous and continuously adjointable operators on a VH-space (Vector Hilbert space in the sense of Loynes) and that are invariant under actions of $*$-semigroups. For such a kernel we obtain two necessary and sufficient boundedness conditions in order for there to exist $*$-representations of the underlying $*$-semigroup on a VH-space linearisation, equivalently, on a reproducing kernel VH-space. We exhibit several situations when the latter boundedness condition is automatically fulfilled. For example, when specialising to the case of Hilbert modules over locally $C^*$-algebras, we show that both boundedness conditions are automatically fulfilled and, consequently, this general approach provides a rather direct proof of the general Stinespring-Kasparov type dilation theorem for completely positive maps on locally $C^*$-algebras and with values adjointable operators on Hilbert modules over locally $C^*$-algebras.

math.OA↗

arXiv2015

Representations of $*$-semigroups associated to invariant kernels with values adjointable operators. I

We consider positive semidefinite kernels valued in the $*$-algebra of adjointable operators on a VE-space (Vector Euclidean space) and that are invariant under actions of $*$-semigroups. A rather general dilation theorem is stated and proved: for these kind of kernels, representations of the $*$-semigroup on either the VE-spaces of linearisation of the kernels or on their reproducing kernel VE-spaces are obtainable. We point out the reproducing kernel fabric of dilation theory and we show that the general theorem unifies many dilation results at the non topological level.

math.FA↗

arXiv2014

Triplets of Closely Embedded Hilbert Spaces

We obtain a general concept of triplet of Hilbert spaces with closed (unbounded) embeddings instead of continuous (bounded) ones. The construction starts with a positive selfadjoint operator $H$, that is called the Hamiltonian of the system, which is supposed to be one-to-one but may not have a bounded inverse, and for which a model is obtained. From this model we get the abstract concept and show that its basic properties are the same with those of the model. Existence and uniqueness results, as well as left-right symmetry, for these triplets of closely embedded Hilbert spaces are obtained. We motivate this abstract theory by a diversity of problems coming from homogeneous or weighted Sobolev spaces, Hilbert spaces of holomorphic functions, and weighted $L^2$ spaces. An application to weak solutions for a Dirichlet problem associated to a class of degenerate elliptic partial differential equations is presented. In this way, we propose a general method of proving the existence of weak solutions that avoids coercivity conditions and Poincaré-Sobolev type inequalities.

math.FA↗

arXiv2014

Interpolation for completely positive maps: numerical solutions

We present certain techniques to find completely positive maps between matrix algebras that take prescribed values on given data. To this aim we describe a semidefinite programming approach and another convex minimization method supported by a numerical example.

math.NA↗