arXiv subjects

Johannes Hahn

Publications and source records attributed to Johannes Hahn.

7 recordsLinked to original sources

arXiv2025

Discrete Single-Parameter Optimal Auction Design

We study the classic single-item auction setting of Myerson, but under the assumption that the buyers' values for the item are distributed over finite supports. Using strong LP duality and polyhedral theory, we rederive various key results regarding the revenue-maximizing auction, including the characterization through virtual welfare maximization and the optimality of deterministic mechanisms, as well as a novel, generic equivalence between dominant-strategy and Bayesian incentive compatibility. Inspired by this, we abstract our approach to handle more general auction settings, where the feasibility space can be given by arbitrary convex constraints, and the objective is a convex combination of revenue and social welfare. We characterize the optimal auctions of such systems as generalized virtual welfare maximizers, by making use of their KKT conditions, and we present an analogue of Myerson's payment formula for general discrete single-parameter auction settings. Additionally, we prove that total unimodularity of the feasibility space is a sufficient condition to guarantee the optimality of auctions with integral allocation rules. Finally, we demonstrate this KKT approach by applying it to a setting where bidders are interested in buying feasible flows on trees with capacity constraints, and provide a combinatorial description of the (randomized, in general) optimal auction.

cs.GT↗

arXiv2021

Photoelectron emission via time and phase-tailored electromagnetic fields

The energy and the angular distributions of photoelectrons are shown to be tunable by choosing the time and the spatial phase structure of the driving fields. These conclusions are derived from quantum mechanical calculations done within a single-active electron model for an atomic target subjected to a combination of laser field and a time-asymmetric THz pulse and/or vortex-laser pulse with a spatially modulated phase of the wavefront.

physics.atom-ph↗

arXiv2018

Gyojas $W$-Graph-Algebra und zelluläre Struktur von Iwahori-Hecke-Algebren

On overview of Coxeter groups $W$, their Iwahori-Hecke algebras $H$, Lusztig's asymptotic algebras $J$, cellular algebras in general and cellularity of $H$ in particular, as well as an introduction to Gyoja's $W$-graph algebra and the $W$-graph decomposition conjecture.

math.RT↗

arXiv2017

On canonical bases and induction of $W$-graphs

A canonical basis in the sense of Lusztig is a basis of a free module over a ring of Laurent polynomials that is invariant under a certain semilinear involution and is obtained from a fixed "standard basis" through a triangular base change matrix with polynomial entries whose constant terms equal the identity matrix. Among the better known examples of canonical bases are the Kazhdan-Lusztig basis of Iwahori-Hecke algebras, Lusztig's canonical basis of quantum groups and the Howlett-Yin basis of induced $W$-graph modules. This paper has two major theoretical goals: First to show that having bases is superfluous in the sense that canonicalisation can be generalized to non-free modules. This construction is functorial in the appropriate sense. The second goal is to show that Howlett-Yin induction of $W$-graphs is well-behaved a functor between module categories of $W$-graph-algebras that satisfies various properties one hopes for when a functor is called "induction", for example transitivity and a Mackey theorem.

math.RT↗

arXiv2017

A Reduction theorem for the $W$-graph decomposition conjecture

Let $W$ be a finite Coxeter group and $Ω$ be its $W$-graph algebra as defined by Gyoja. The author's previous paper \cite{hahn2016wgraphs} considered this algebra in some detail, proposed, and proved in some small cases the $W$-graph decomposition conjecture. The purpose of the current paper is to prove a reduction theorem for (a slightly stronger version of) that conjecture to indecomposable Coxeter groups in the sense that the conjecture is true for $W=W_1\times W_2$ if it holds for $W_1$ and $W_2$.

math.RT↗

arXiv2016

W-graphs and Gyoja's W-graph algebra

Let $(W,S)$ be a finite Coxeter group. Kazhdan and Lusztig introduced the concept of $W$-graphs and Gyoja proved that every irreducible representation of the Iwahori-Hecke algebra $H(W,S)$ can be realized as a $W$-graph. Gyoja defined an auxiliary algebra for this purpose which -- to the author's best knowledge -- was never explicitly mentioned again in the literature after Gyoja's proof (although the underlying ideas were reused). The purpose of this paper is to resurrect this $W$-graph algebra and study its structure and its modules. A new explicit description of it as a quotient of a certain path algebra is given. A general conjecture is proposed that -- if it turns out to be true -- would imply strong restrictions on the structure of $W$-graphs. This conjecture is then proven for Coxeter groups of type $I_2(m)$, $B_3$ and $A_1$ -- $A_4$.

math.RT↗

arXiv2010

An interesting proof of the nonexistence continuous bijection between $\mathbb{R}^n$ and $\mathbb{R}^2$ for $n\neq 2$}

In this article it is shown that there is no continuous bijection from $\mathbb{R}^n$ onto $\mathbb{R}^2$ for $n\neq 2$ by an elementary method. This proof is based on showing that for any cardinal number $β\leq 2^{\aleph_0}$, there is a partition of $R^n$ ($n\geq 3$) into $β$ arcwise connected dense subsets.

math.GN↗