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A. J. Ostaszewski

Publications and source records attributed to A. J. Ostaszewski.

At least 19 recordsLinked to original sources
arXiv2024

Homomorphisms from Functional Equations: The Goldie Equation, II

In this sequel to arXiv1407.4089 by the second author, we extend to multi-dimensional (or infinite-dimensional) settings the Goldie equation arising in the general regular variation of `General regular variation, Popa groups and quantifier weakening', J. Math. Anal. Appl. 483 (2020) 123610, 31 pp. (arXiv1901.05996). We extend the work there in two directions. First, we algebraicize the theory, by systematic use of certain groups -- the Popa groups arising in earlier work by Popa, and their relatives the Javor groups. Secondly, we extend from the original context on the real line to multi-dimensional (or infinite-dimensional) settings.

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arXiv2024

The Goldie Equation: III. Homomorphisms from Functional Equations

This is the second of three sequels to arXiv1407.4089 -- the third of the resulting quartet -- concerning the real-valued continuous solutions of the multivariate Goldie functional equation (GFE) below of Levi-Civita type. Following on from the preceding paper arXiv1910.05816, in which these solutions are described explicitly, here we characterize (GFE) as expressing homomorphy (in all but some exceptional "improper" cases) between multivariate Popa groups, defined and characterized earlier in the sequence. The group operation involves a form of affine addition (with local scalar acceleration) similar to the circle operation of ring theory. We show the affine action in (GFE) may be replaced by general continuous acceleration yielding a functional equation (GGE) which it emerges has the same solution structure as (GFE). The final member of the sequence arXiv2105.07794 considers the richer framework of a Banach algebra which allows vectorial acceleration, giving the closest possible similarity to the circle operation.

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arXiv2021

The Gołąb-Schinzel and Goldie functional equations in Banach algebras

We are concerned below with the characterization in a unital commutative real Banach algebra $\mathbb{A}$ of continuous solutions of the Gołąb-Schinzel functional equation (below), the general Popa groups they generate and the associated Goldie functional equation. This yields general structure theorems involving both linear and exponential homogeneity in $\mathbb{A}$ for both these functional equations and also explict forms, in terms of the recently developed theory of multi-Popa groups [BinO3,4], both for the ring $C[0,1]$ and for the case of $\mathbb{R}^{d}$ with componentwise product, clarifying the context of recent developments in [RooSW]. The case $\mathbb{A}=\mathbb{C}$ provides a new viewpoint on continuous complex-valued solutions of the primary equation by distinguishing analytic from real-analytic ones.

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arXiv2020

Extremes and regular variation

We survey the connections between extreme-value theory and regular variation, in one and higher dimensions, from the algebraic point of view of our recent work on Popa groups.

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arXiv2019

Subdominant eigenvalue location and the robustness of Dividend Policy Irrelevance

This paper, on subdominant eigenvalue location of a bordered diagonal matrix, is the mathematical sequel to an accounting paper by Gao, Ohlson, Ostaszewski \cite{GaoOO}. We explore the following characterization of dividend-policy irrelevance (DPI) to equity valuation in a multi-dimensional linear dynamics framework $L$: DPI occurs under $L$ when discounting the expected dividend stream by a constant interest rate iff that rate is equal to the dominant eigenvalue of the canonical principal submatrix $A$ of $L.$ This is justifiably the `latent' (or gross) rate of return, since the principal submatrix relates the state variables to each other but with dividend retention. We find that DPI reduces to the placement of the maximum eigenvalue of $L$ between the dominant and subdominant eigenvalues of $A.$ We identify a special role, and a lower bound, for the coefficient measuring the year-on-year dividend-on-dividend sensitivity in achieving robust equity valuation (independence of small variations in the dividend policy).

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arXiv2019

Optimal forward contract design for inventory: a value-of-waiting analysis

A classical inventory problem is studied from the perspective of embedded options, reducing inventory-management to the design of optimal contracts for forward delivery of stock (commodity). Financial option techniques à la Black-Scholes are invoked to value the additional `option to expand stock'. A simplified approach which ignores distant time effects identifies an optimal `time to deliver' and an optimal `amount to deliver' for a production process run in continuous time modelled by a Cobb-Douglas revenue function. Commodity prices, quoted in initial value terms, are assumed to evolve as a geometric Brownian process with positive drift. Expected revenue maximization identifies an optimal `strike price' for the expansion option to be exercised, and uncovers the underlying martingale in a truncated (censored) commodity price. The paper establishes comparative statics of the censor in terms of drift and volatility, and uses asymptotic approximation for a tractable analysis of the optimal timing.

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arXiv2019

Beyond Haar and Cameron-Martin: the Steinhaus support

Motivated by a Steinhaus-like interior-point property involving the Cameron-Martin space of Gaussian measure theory, we study a group-theoretic analogue, the Steinhaus triple $(H,G,μ)$, and construct a Steinhaus support, a Cameron-Martin-like subset, $H(μ)$ in any Polish group $G$ corresponding to `sufficiently subcontinuous' measures $μ$, in particular for `Solecki-type' reference measures.

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arXiv2019

Beyond Erdős-Kunen-Mauldin: Singular sets with shift-compactness properties

The Kestelman-Borwein-Ditor Theorem asserts that a non-negligible subset of $\mathbb{R}$ which is Baire (=has the Baire property, BP) or measurable is shift-compact: it contains some subsequence of any null sequence to within translation by an element of the set. Effective proofs are recognized to yield (i) analogous category and Haar-measure metrizable generalizations for Baire groups and locally compact groups respectively, and (ii) permit under $V=L$ construction of co-analytic shift-compact subsets of R with singular properties, e.g. being concentrated on $\mathbb{Q}$, the rationals.

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arXiv2019

Sequential regular variation: extensions of Kendall's theorem

Regular variation is a continuous-parameter theory; we work in a general setting, containing the existing Karamata, Bojanic-Karamata/de Haan and Beurling theories as special cases. We give sequential versions of the main theorems, that is, with sequential rather than continuous limits. This extends the main result, a theorem of Kendall's (which builds on earlier work of Kingman and Croft), to the general setting.

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arXiv2019

General Regular Variation, Popa Groups and Quantifier Weakening

We introduce general regular variation, a theory of regular variation containing the existing Karamata, Bojanic-Karamata/de Haan and Beurling theories as special cases. The unifying theme is the Popa groups of our title viewed as locally compact abelian ordered topological groups, together with their Haar measure and Fourier theory. The power of this unified approach is shown by the simplification it brings to the whole area of quantifier weakening, so important in this field.

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arXiv2018

The Steinhaus-Weil property: its converse, subcontinuity and Solecki amenability

The Steinhaus-Weil theorem that concerns us here is the simple, or classical, `interior-points' property -- that in a Polish topological group a non-negligible set B has the identity as an interior point of $BB^{-1}$. There are various converses; the one that mainly concerns us is due to Simmons and Mospan. Here the group is locally compact, so we have a Haar reference measure $η$. The Simmons-Mospan theorem states that a (regular Borel) measure has such a Steinhaus-Weil property if and only if it is absolutely continuous with respect to the Haar measure. In Part I (Propositions 1-7, Theorems 1-4) we exploit the connection between the interior-points property and a selective form of infinitesimal invariance afforded by a certain family of selective reference measures $σ$, drawing on Solecki's amenability at 1 (and using Fuller's notion of subcontinuity). In Part II (Propositions 8, 9, Theorems 5, 6) we develop a number of relatives of the Simmons-Mospan theorem. In Part III (Theorems 7, 8) we link this with topologies of Weil type. We close in Part IV with Propositions 12-13 and Theorem B -- concerning the `composite interior-point' property (of $AB^{-1}$) and the Borell `relative interior-point' property (relative to the Cameron-Martin space) -- and complements.

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arXiv2018

Set Theory and the Analyst

This survey is motivated by specific questions arising in the similarities and contrasts between (Baire) category and (Lebesgue) measure -- category-measure duality and non-duality, as it were. The bulk of the text is devoted to a summary, intended for the working analyst, of the extensive background in set theory and logic needed to discuss such matters: to quote from the Preface of Kelley [Kel]: "what every young analyst should know".

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arXiv2017

Variants on the Berz sublinearity theorem

We consider variants on the classical Berz sublinearity theorem, using only DC, the Axiom of Dependent Choices, rather than AC, the Axiom of Choice which Berz used. We consider thinned versions, in which conditions are imposed on only part of the domain of the function -- results of quantifier-weakening type. There are connections with classical results on subadditivity. We close with a discussion of the extensive related literature.

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arXiv2017

Additivity, subadditivity and linearity: automatic continuity and quantifier weakening

We study the interplay between additivity (as in the Cauchy functional equation), subadditivity and linearity. We obtain automatic continuity results in which additive or subadditive functions, under minimal regularity conditions, are continuous and so linear. We apply our results in the context of quantifier weakening in the theory of regular variation completing our programme of reducing the number of hard proofs there to zero.

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arXiv2017

Beyond Lebesgue and Baire IV: Density topologies and a converse Steinhaus-Weil Theorem

The theme here is category-measure duality, in the context of a topological group. One can often handle the (Baire) category case and the (Lebesgue, or Haar) measure cases together, by working bi-topologically: switching between the original topology and a suitable refinement (a density topology). This prompts a systematic study of such density topologies, and the corresponding $σ$-ideals of negligibles. Such ideas go back to Weil's classic book, and to Hashimoto's ideal topologies. We make use of group norms, which cast light on the interplay between the group and measure structures. The Steinhaus-Weil interior-points theorem (`on $AA^{-1}$') plays a crucial role here; so too does its converse, the Simmons-Mospan theorem.

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arXiv2016

Category-measure duality: convexity, mid-point convexity and Berz sublinearity

Category-measure duality concerns applications of Baire-category methods that have measure-theoretic analogues. The set-theoretic axiom needed in connection with the Baire category theorem is the Axiom of Dependent Choice DC rather than the Axiom of Choice AC. Berz used the Hahn-Banach Theorem over $\mathbb{Q}$ to prove that the graph of a measurable sublinear function that is $\mathbb{Q}_{+}$-homogeneous consists of two half-lines through the origin. We give a category form of the Berz theorem. Our proof is simpler than that of the classical measure-theoretic Berz theorem, our result contains Berz's theorem rather than simply being an analogue of it, and we use only DC rather than AC. Furthermore, the category form easily generalizes: the graph of a Baire sublinear function defined on a Banach space is a cone. The results are seen to be of automatic-continuity type. We use Christensen Haar null sets to extend the category approach beyond the locally compact setting where Haar measure exists. We extend Berz's result from Euclidean to Banach spaces, and beyond. Passing from sublinearity to convexity, we extend the Bernstein-Doetsch theorem and related continuity results, allowing our conditions to be `local' -- holding off some exceptional set.

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arXiv2016

Beurling moving averages and approximate homomorphisms

The theory of regular variation, in its Karamata and Bojanić-Karamata/de Haan forms, is long established and makes essential use of homomorphisms. Both forms are subsumed within the recent theory of Beurling regular variation, developed further here, especially certain moving averages occurring there. Extensive use of group structures leads to an algebraicization not previously encountered here, and to the approximate homomorphisms of the title. Dichotomy results are obtained: things are either very nice or very nasty. Quantifier weakening is extended, and the degradation resulting from working with limsup and liminf, rather than assuming limits exist, is studied.

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