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SPM Bulletin 22  [PDF]
Boaz Tsaban
Mathematics , 2007,
Abstract: Contents: 2. Invited contribution: Ultrafilters and small sets 3. Research announcements 3.1. Inverse Systems and I-Favorable Spaces 3.2. Combinatorial and hybrid principles for sigma-directed families of countable sets modulo finite 3.3. A dichotomy characterizing analytic digraphs of uncountable Borel chromatic number in any dimension 3.4. A dichotomy characterizing analytic digraphs of uncountable Borel chromatic number in any dimension 3.5. Large continuum, oracles 3.6. Borel hierarchies in infinite products of Polish spaces 3.7. A game for the Borel functions 3.8. On some problems in general topology 4. Problem of the Issue
A dichotomy for Borel functions  [PDF]
Marcin Sabok
Mathematics , 2008,
Abstract: The dichotomy discovered by Solecki in \cite{Sol} states that any Baire class 1 function is either $\sigma$-continuous or "includes" the Pawlikowski function $P$. The aim of this paper is to give an argument which is simpler than the original proof of Solecki and gives a stronger statement: a dichotomy for all Borel functions.
The Dichotomy of List Homomorphisms for Digraphs  [PDF]
Pavol Hell,Arash Rafiey
Computer Science , 2010,
Abstract: The Dichotomy Conjecture for constraint satisfaction problems has been verified for conservative problems (or, equivalently, for list homomorphism problems) by Andrei Bulatov. An earlier case of this dichotomy, for list homomorphisms to undirected graphs, came with an elegant structural distinction between the tractable and intractable cases. Such structural characterization is absent in Bulatov's classification, and Bulatov asked whether one can be found. We provide an answer in the case of digraphs; the technique will apply in a broader context. The key concept we introduce is that of a digraph asteroidal triple (DAT). The dichotomy then takes the following form. If a digraph H has a DAT, then the list homomorphism problem for H is NP-complete; and a DAT-free digraph H has a polynomial time solvable list homomorphism problem. DAT-free graphs can be recognized in polynomial time.
Borel chromatic number of closed graphs  [PDF]
Dominique Lecomte,Miroslav Zeleny
Mathematics , 2014,
Abstract: We construct, for each countable ordinal $\xi$, a closed graph with Borel chromatic number two and Baire class $\xi$ chromatic number $\aleph\_0$.
On the reduction of the CSP dichotomy conjecture to digraphs  [PDF]
Jakub Bulin,Dejan Delic,Marcel Jackson,Todd Niven
Computer Science , 2013,
Abstract: It is well known that the constraint satisfaction problem over general relational structures can be reduced in polynomial time to digraphs. We present a simple variant of such a reduction and use it to show that the algebraic dichotomy conjecture is equivalent to its restriction to digraphs and that the polynomial reduction can be made in logspace. We also show that our reduction preserves the bounded width property, i.e., solvability by local consistency methods. We discuss further algorithmic properties that are preserved and related open problems.
Oriented paths in n-chromatic digraphs  [PDF]
Rajai Nasser
Mathematics , 2011,
Abstract: In this thesis, we try to treat the problem of oriented paths in n-chromatic digraphs. We first treat the case of antidirected paths in 5-chromatic digraphs, where we explain El-Sahili's theorem and provide an elementary and shorter proof of it. We then treat the case of paths with two blocks in n-chromatic digraphs with n greater than 4, where we explain the two different approaches of Addario-Berry et al. and of El-Sahili. We indicate a mistake in Addario-Berry et al.'s proof and provide a correction for it.
The $b$-Chromatic Number of Certain Graphs and Digraphs  [PDF]
Johan Kok,Naduvath Sudev
Mathematics , 2015,
Abstract: The studies on $b$-chromatic number attracted much interest since its introduction. In this paper, we discuss the $b$-chromatic number of certain classes of graphs and digraphs. The notion of a new general family of graphs called the Chithra graphs of a graph $G$ are also introduced.
An uncountable family of regular Borel measures on certain path spaces of Lipschitz functions  [PDF]
Richard L. Baker
Mathematics , 2007,
Abstract: We present a general method of constructing an uncountable family of regular Borel measures on certain path spaces of Lipschitz functions having fixed Lipschitz constants. We use this method to give a definition of Lebesgue measure and uniform probability measre on these spaces.
A dichotomy theorem for the generalized Baire space and elementary embeddability at uncountable cardinals  [PDF]
Dorottya Sziráki,Jouko V??n?nen
Mathematics , 2015,
Abstract: We consider the following dichotomy for $\Sigma^0_2$ finitary relations $R$ on analytic subsets of the generalized Baire space for $\kappa$: either all $R$-independent sets are of size at most $\kappa$, or there is a $\kappa$-perfect $R$-independent set. This dichotomy is the uncountable version of a result found in (W. Kubi\'s, Proc. Amer. Math. Soc. 131 (2003), no 2.:619--623) and in (S. Shelah, Fund. Math. 159 (1999), no. 1:1--50). We prove that the above statement holds assuming $\Diamond_\kappa$ and the set theoretical hypothesis $I^-(\kappa)$, which is the modification of the hypothesis $I(\kappa)$ suitable for limit cardinals. When $\kappa$ is inaccessible, or when the relation $R$ is closed, the assumption $\Diamond_\kappa$ is not needed. We obtain as a corollary the uncountable version of a result by G. S\'agi and the first author (Log. J. IGPL 20 (2012), no. 6:1064--1082) about the $\kappa$-sized models of a $\Sigma^1_1(L_{\kappa^+\kappa})$-sentence when considered up to isomorphism, or elementary embeddability, by elements of a $K_\kappa$ subset of ${}^\kappa\kappa$. The role of elementary embeddings can be replaced by a more general notion that also includes embeddings, as well as the maps preserving $L_{\lambda\mu}$ for $\omega\leq\mu\leq\lambda\leq\kappa$ and the finite variable fragments of these logics.
The conjugacy problem for automorphism groups of homogeneous digraphs  [PDF]
Samuel Coskey,Paul Ellis
Mathematics , 2015,
Abstract: We decide the Borel complexity of the conjugacy problem for automorphism groups of countable homogeneous digraphs. Many of the homogeneous digraphs, as well as several other homogeneous structures, have already been addressed in previous articles. In this article we complete the program, and establish a dichotomy theorem that this complexity is either the minimum or the maximum among relations which are classifiable by countable structures. We also discuss the possibility of extending our results beyond graphs to more general classes of countable homogeneous structures.
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