Dariusz Cichoń

Extending positive definiteness

The main result of this paper gives criteria for extendibility of mappings defined on symmetric subsets of *-semigroups to positive definite ones. By specifying the mappings in question we obtain new solutions of relevant issues in harmonic analysis concerning truncations of some important multivariate moment problems, like complex, two-sided complex and multidimensional trigonometric moment problems. In addition, unbounded subnormality and existence of unitary power dilation of several contractions is treated as an application of our general scheme.

Three term recurrence relation modulo ideal and orthogonality of polynomials of several variables

Orthogonality of polynomials in several variables with respect to a positive Borel measure supported on an algebraic set is the main theme of this paper. As a step towards this goal quasi-orthogonality with respect to a non-zero Hermitian linear functional is studied in detail; this occupies a substantial part of the paper. Therefore necessary and sufficient conditions for quasi-orthogonality in terms of the three term recurrence relation modulo a polynomial ideal are accompanied with a thorough discussion. All this enables us to consider orthogonality in full generality. Consequently, a class of simple objects missing so far, like spheres, is included. This makes it important to search for results on existence of measures representing orthogonality on algebraic sets; a general approach to this problem fills up the three final sections.

Selfadjointness of integral and matrix operators

Criteria for selfadjointness of integral operators, including matrix operators, based on our earlier domination results are established. The range of applicability is elucidated by carefully chosen examples, not covered by previously known criteria.

À la recherche de la preuve perdue: a simple proof of the Ficken theorem

Abstract A new proof of the Ficken criterion is given together with a comment concerning the known proofs and related results