arXiv subjects

Ohad Drucker

Publications and source records attributed to Ohad Drucker.

5 recordsLinked to original sources

arXiv2025

Synthesis of Infinite State Systems

The classical Church synthesis problem, solved by Buchi and Landweber, treats the synthesis of finite state systems. The synthesis of infinite state systems, on the other hand, has only been investigated few times since then, with no complete or systematic solution. We present a systematic study of the synthesis of infinite state systems. The main step involves the synthesis of MSO-definable parity games, which is, finding MSO-definable uniform memoryless winning strategies for these games.

cs.LO↗

arXiv2016

Borel Canonization of Analytic Sets with Borel Sections

Given an analytic equivalence relation, we tend to wonder whether it is Borel. When it is non Borel, there is always the hope it will be Borel on a "large" set -- nonmeager or of positive measure. That has led Kanovei, Sabok and Zapletal to ask whether every proper $σ$ ideal satisfies the following property: given $E$ an analytic equivalence relation with Borel classes, there exists a set $B$ which is Borel and $I$-positive such that $E\restriction_{B}$ is Borel. We propose a related problem -- does every proper $σ$ ideal satisfy: given $A$ an analytic subset of the plane with Borel sections, there exists a set $B$ which is Borel and $I$-positive such that $A\cap(B\timesω^ω)$ is Borel. We answer positively when a measurable cardinal exists, and negatively in $L$, where no proper $σ$ ideal has that property. Assuming $ω_{1}$ is inaccessible to the reals but not Mahlo in $L$, we construct a ccc $σ$ ideal $I$ not having this property -- in fact, forcing with $I$ adds a non Borel section to a certain analytic set with Borel sections, and a non Borel class to a certain analytic equivalence relation with Borel classes. Various counterexamples are given for the case of a $\mathbf{Δ_{2}^{1}}$ equivalence relation as well as for the case of an improper ideal.

math.LO↗

arXiv2016

Perfect Set Theorems for Equivalence Relations with $I$ - small classes

A classical theorem due to Mycielski states that an equivalence relation $E$ having the Baire property and meager equivalence classes must have a perfect set of pairwise inequivalent elements. We consider equivalence relations with $I$-small equivalence classes, where $I$ is a proper $σ$-ideal, and ask whether they have a perfect set of pairwise inequivalent elements. We give a positive answer for $E$ universally Baire. We show that the answer for $E$ $\mathbf{Δ_{2}^{1}}$ is independent of $ZFC$, and find set theoretic assumptions equivalent to it when $I$ is the countable ideal. For equivalence relations which are $\mathbf{Σ^1_2}$ and with meager classes, we show that a perfect set of pairwise inequivalent elements exists whenever a Cohen real over $L[z]$ exists for any real $z$ -- which strengthens Mycielski's theorem. A few comments are made about $σ$-ideals generated by $Π_{1}^{1}$ and orbit equivalence relations.

math.LO↗

arXiv2016

Measurability and Perfect Set Theorems for Equivalence Relations with Small Classes

We ask whether $\mathbf{Δ^1_2}$ or $\mathbf{Σ^1_2}$ equivalence relations with $I$-small classes for $I$ a $σ$-ideal must have perfectly many classes. We show that for a wide class of ccc $σ$-ideals, a positive answer for $\mathbf{Δ^1_2}$ equivalence relations is equivalent to the $I$-measurability of $\mathbf{Δ^1_2}$ sets. However, the analogous statement for $\mathbf{Σ^1_2}$ equivalence relations is false: $\mathbf{Σ^1_2}$ equivalence relations with meager classes have a perfect set of pairwise inequivalent elements if and only if $\mathbf{Δ^1_2}$ sets have the Baire property.

math.LO↗

arXiv2015

Hjorth Analysis of General Polish Group Actions

Hjorth has introduced a Scott analysis for general Polish group actions, and has asked whether his notion of rank satisfies a boundedness principle similar to the one of Scott rank - namely, the orbit equivalence relation is Borel if and only if Hjorth ranks are bounded. We present the principles of Hjorth analysis and Hjorth rank, and answer Hjorth's question positively. From that we get a positive answer to a conjecture due to Hjorth - for every limit ordinal $α$ , the set of elements whose orbit is of complexity less than $α$ is a Borel set. We then show Nadel's theorem for Hjorth rank - the rank of $x$ is no more than $ω_{1}^{ck(x)}$.

math.LO↗