arXiv subjects

Imed Zaguia

Publications and source records attributed to Imed Zaguia.

At least 19 recordsLinked to original sources

arXiv2026

Gallai Decomposition of Ordered Groups: Subgroups, Quotients, and the $N$-free Case

We study Gallai decomposition for groups equipped with two-sided invariant partial orders. The key algebraic step extends to arbitrary binary relations compatible with the group operation: if all left and right translations preserve a binary relation $ρ$, then every least strong module $S_ρ(\e,g)$, $g\ne\e$, is a subgroup. For a partial order this subgroup is convex. Thus the robust modules through the identity of an ordered group form a canonical chain of convex subgroups, with each canonical factor $H/H^-$ prime, totally ordered, or equality-ordered. We characterize exactly the subgroups that are modules, show that they form a complete sublattice of the subgroup lattice, establish overlap and inheritance results for arbitrary subgroups, and prove compatibility with quotients by normal strong subgroups. For $N$-free ordered groups the prime factors disappear. Using the robust-module decomposition of cographs, we characterize all two-sided invariant $N$-free partial orders by reduced admissible two-coloured subgroup chains, with totally ordered and equality-ordered canonical factors; the order is determined by the first nontrivial factor of each element. We also determine how the canonical decomposition restricts to arbitrary subgroups, characterize finite width and prove width divisibility for subgroups, and show that every reduced two-coloured chain is realized by an $N$-free ordered abelian group whose canonical factors are isomorphic to $\mathbb Z$.

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arXiv2026

The structure of interval orders with no infinite antichain

We prove that if $G=(V,E)$ is a nonprime graph with either no infinite independent set or no infinite clique, then every vertex of $G$ belongs to a maximal strong module distinct from $V$. In particular, $G$ admits a Gallai decomposition. As a consequence, we obtain that every interval order $P$ with no infinite antichain admits a Gallai decomposition. That is, $P$ is a lexicographical sum of interval orders distinct from $P$ indexed by either a chain, an antichain, or a prime interval order. Next, we prove that every prime interval order with no infinite antichain is at most countable and does not embed a copy of the chain of rational numbers. Finally, for each countable ordinal $α$, we construct a well-quasi-ordered prime interval order $P_α$ whose chain of maximal antichains has Hausdorff rank $α$.

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arXiv2025

Cops and Robbers, Clique Covers, and Induced Cycles

We consider the Cops and Robbers game played on finite simple graphs. In a graph $G$, the number of cops required to capture a robber in the Cops and Robbers game is denoted by $c(G)$. For all graphs $G$, $c(G) \leq α(G) \leq θ(G)$ where $α(G)$ and $θ(G)$ are the independence number and clique cover number respectively. In 2022 Turcotte asked if $c(G) < α(G)$ for all graphs with $α(G) \geq 3$. Recently, Char, Maniya, and Pradhan proved this is false, at least when $α= 3$,by demonstrating the compliment of the Shrikhande graph has cop number and independence number $3$. We prove, using random graphs, the stronger result that for all $k\geq 1$ there exists a graph $G$ such that $c(G) = α(G) = θ(G) = k$. Next, we consider the structure of graphs with $c(G) = θ(G) \geq 3$. We prove, using structural arguments, that any graphs $G$ which satisfies $c(G) = θ(G) = k \geq 3$ contain induced cycles of all lengths $3\leq t \leq k+1$. This implies all perfect graphs $G$ with $α(G)\geq 4$ have $c(G) < α(G)$. Additionally,we discuss if typical triangle-free and $C_4$-free graphs will have $c(G) < α(G)$.

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arXiv2023

The Aharoni--Korman conjecture for $N$-free posets with no infinite antichain

We give a necessary and sufficient condition for a $P_4$-free graph to be a cograph. This allows us to obtain a simple proof of the fact that finite $P_4$-free graphs are finite cographs. We also prove that $N$-free chain complete posets and $N$-free posets with no infinite antichains are series-parallel. As a consequence, we obtain that every $N$-free poset with no infinite antichain has a chain and a partition into antichains so that each part intersects the chain. This answers a conjecture of Aharoni and Korman (Order \textbf{9} (1992) 245--253) in this case.

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arXiv2023

The Aharoni--Korman conjecture for posets whose incomparability graph is locally finite

Aharoni and Korman (Order 9 (1992) 245--253) have conjectured that every ordered set without infinite antichains possesses a chain and a partition into antichains so that each part intersects the chain. The conjecture is verified for posets whose incomparability graph is locally finite. It follows that the conjecture is true for $(3 + 1)$-free posets with no infinite antichains.

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arXiv2022

Hereditary classes of ordered sets of width at most two

This paper is a contribution to the study of hereditary classes of relational structures, these classes being quasi-ordered by embeddability. It deals with the specific case of ordered sets of width two and the corresponding bichains and incomparability graphs. Several open problems about hereditary classes of relational structures which have been considered over the years have positive answer in this case. For example, well-quasi-ordered hereditary classes of finite bipartite permutation graphs, respectively finite 321-avoiding permutations, have been characterized by Korpelainen, Lozin and Mayhill, respectively by Albert, Brignall, Ruškuc and Vatter. We provide another proof of the results mentioned above. It is based on the existence of a countable universal poset of width two, obtained by the first author in 1978, his notion of multichainability (1978) (a kind of analog to letter-graphs), and metric properties of incomparability graphs. Using Laver's theorem (1971) on better-quasi-ordering (bqo) of countable chains we prove that a wqo hereditary class of finite or countable bipartite permutation graphs is necessarily bqo. This gives a positive answer to a conjecture of Nash-Williams (1965) in this case. We extend a previous result of Albert et al. by proving that if a hereditary class of finite, respectively countable, bipartite permutation graphs is wqo, respectively bqo, then the corresponding hereditary classes of posets of width at most two and bichains are wqo, respectively bqo. Several notions of labelled wqo are also considered. We prove that they are all equivalent in the case of bipartite permutation graphs, posets of width at most two and the corresponding bichains. We characterize hereditary classes of finite bipartite permutation graphs which remain wqo when labels from a wqo are added.

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arXiv2022

Minimal prime ages, words and permutation graphs

This paper is a contribution to the study of hereditary classes of finite graphs. We classify these classes according to the number of prime structures they contain. We consider such classes that are \emph{minimal prime}: classes that contain infinitely many primes but every proper hereditary subclass contains only finitely many primes. We give a complete description of such classes. In fact, each one of these classes is a well-quasi-ordered (w.q.o) age and there are uncountably many of them. Eleven of these ages are almost multichainable; they remain w.q.o when labels in a w.q.o are added, hence have finitely many bounds. Five ages among them are exhaustible. Among the remaining ones, only countably many remain w.q.o when one label is added, and these have finitely many bounds (except for the age of the infinite path and its complement). The others have infinitely many bounds. Except for six examples, members of these ages we characterize are permutation graphs. In fact, every age which is not among the eleven ones is the age of a graph associated to a uniformly recurrent word on the integers. A description of minimal prime classes of posets and bichains is also provided. Our results support the conjecture that if a hereditary class of finite graphs does not remain w.q.o when adding labels from a w.q.o set to these graphs, then it is not w.q.o if we add just two constants to each of these graphs Our description of minimal prime classes uses a description of minimal prime graphs \cite{pouzet-zaguia2009} and previous work by Sobrani \cite{sobranithesis, sobranietat} and the authors \cite{oudrar, pouzettr} on properties of uniformly recurrent words and the associated graphs. The completeness of our description is based on classification results of Chudnovsky, Kim, Oum and Seymour \cite{chudnovsky} and Malliaris and Terry \cite {malliaris}.

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arXiv2022

Minimal prime ages, words and permutation graphs Extended abstract

math.CO↗

arXiv2022

Graphs containing finite induced paths of unbounded length

The age $\mathcal{A}(G)$ of a graph $G$ (undirected and without loops) is the collection of finite induced subgraphs of $G$, considered up to isomorphy and ordered by embeddability. It is well-quasi-ordered (wqo) for this order if it contains no infinite antichain. A graph is \emph{path-minimal} if it contains finite induced paths of unbounded length and every induced subgraph $G'$ with this property embeds $G$. We construct $2^{\aleph_0}$ path-minimal graphs whose ages are pairwise incomparable with set inclusion and which are wqo. Our construction is based on uniformly recurrent sequences and lexicographical sums of labelled graphs.

math.CO↗

arXiv2022

Metric properties of incomparability graphs with an emphasis on paths

We describe some metric properties of incomparability graphs. We consider the problem of the existence of infinite paths, either induced or isometric, in the incomparability graph of a poset. Among other things, we show that if the incomparability graph of a poset is connected and has infinite diameter, then it contains an infinite induced path. Furthermore, if the diameter of the set of vertices of degree at least $3$ is infinite, then the graph contains as an induced subgraph either a comb or a kite.

math.CO↗

arXiv2021

Permutations Avoiding Certain Partially-ordered Patterns

A permutation $π$ contains a pattern $σ$ if and only if there is a subsequence in $π$ with its letters are in the same relative order as those in $σ$. Partially ordered patterns (POPs) provide a convenient way to denote patterns in which the relative order of some of the letters does not matter. This paper elucidates connections between the avoidance sets of a few POPs with other combinatorial objects, directly answering five open questions posed by Gao and Kitaev \cite{gao-kitaev-2019}. This was done by thoroughly analysing the avoidance sets and developing recursive algorithms to derive these sets and their corresponding combinatorial objects in parallel, which yielded a natural bijection. We also analysed an avoidance set whose simple permutations are enumerated by the Fibonacci numbers and derived an algorithm to obtain them recursively.

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arXiv2020

Greedy balanced pairs in $N$-free ordered sets

An $α$-greedy balanced pair in an ordered set $P=(V,\leq)$ is a pair $(x,y)$ of elements of $V$ such that the proportion of greedy linear extensions of $P$ that put $x$ before $y$ among all greedy linear extensions is in the real interval $[α, 1-α]$. We prove that every $N$-free ordered set which is not totally ordered has a $\frac{1}{2}$-greedy balanced pair.

math.CO↗

arXiv2018

Interval orders, semiorders and ordered groups

We prove that the order of an ordered group is an interval order if and only if it is a semiorder. Next, we prove that every semiorder is isomorphic to a collection $\mathcal J$ of intervals of some totally ordered abelian group, these intervals being of the form $[x, x+ α[$ for some positive $α$. We describe ordered groups such that the ordering is a semiorder and we introduce threshold groups generalizing totally ordered groups. We show that the free group on finitely many generators and the Thompson group $\mathbb F$ can be equipped with a compatible semiorder which is not a weak order. On another hand, a group introduced by Clifford cannot.

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arXiv2017

The 1/3-2/3 Conjecture for ordered sets whose cover graph is a forest

A balanced pair in an ordered set $P=(V,\leq)$ is a pair $(x,y)$ of elements of $V$ such that the proportion of linear extensions of $P$ that put $x$ before $y$ is in the real interval $[1/3, 2/3]$. We define the notion of a good pair and claim any ordered set that has a good pair will satisfy the conjecture and furthermore every ordered set which is not totally ordered and has a forest as its cover graph has a good pair.

math.CO↗

arXiv2016

Iterated Arc Graphs

The arc graph $δ(G)$ of a digraph $G$ is the digraph with the set of arcs of $G$ as vertex-set, where the arcs of $δ(G)$ join consecutive arcs of $G$. In 1981, Poljak and Rödl characterised the chromatic number of $δ(G)$ in terms of the chromatic number of $G$ when $G$ is symmetric (i.e., undirected). In contrast, directed graphs with equal chromatic numbers can have arc graphs with distinct chromatic numbers. Even though the arc graph of a symmetric graph is not symmetric, we show that the chromatic number of the iterated arc graph $δ^k(G)$ still only depends on the chromatic number of $G$ when $G$ is symmetric.

math.CO↗

arXiv2014

Some inequalities for orderings of acyclic digraphs

Let $D=(V,A)$ be an acyclic digraph. For $x\in V$ define $e_{_{D}}(x)$ to be the difference of the indegree and the outdegree of $x$. An acyclic ordering of the vertices of $D$ is a one-to-one map $g: V \rightarrow [1,|V|] $ that has the property that for all $x,y\in V$ if $(x,y)\in A$, then $g(x) < g(y)$. We prove that for every acyclic ordering $g$ of $D$ the following inequality holds: \[\sum_{x\in V} e_{_{D}}(x)\cdot g(x) ~\geq~ \frac{1}{2} \sum_{x\in V}[e_{_{D}}(x)]^2~.\] The class of acyclic digraphs for which equality holds is determined as the class of comparbility digraphs of posets of order dimension two.

math.CO↗

arXiv2014

Pairs of orthogonal countable ordinals

We characterize pairs of orthogonal countable ordinals. Two ordinals $α$ and $β$ are orthogonal if there are two linear orders $A$ and $B$ on the same set $V$ with order types $α$ and $β$ respectively such that the only maps preserving both orders are the constant maps and the identity map. We prove that if $α$ and $β$ are two countable ordinals, with $α\leq β$, then $α$ and $β$ are orthogonal if and only if either $ω+ 1\leq α$ or $α=ω$ and $β< ωβ$.

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arXiv2012

The 1/3-2/3 conjecture for $N$-free ordered sets

A balanced pair in a finite ordered set $P=(V,\leq)$ is a pair $(x,y)$ of elements of $V$ such that the proportion of linear extensions of $P$ that put $x$ before $y$ is in the real interval $[1/3, 2/3]$. We prove that every finite $N$-free ordered set which is not totally ordered has a balanced pair.

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