On separating sets of words. V.
On some Interconnections Between Combinatorial Optimization and Extremal Graph Theory
On some packing problem related to dynamic storage allocation
On some problems related to palindrome closure
In this paper, we solve some open problems related to (pseudo)palindrome closure operators and to the infinite words generated by their iteration, that is, standard episturmian and pseudostandard words. We show that if ϑ is an involutory antimorphism of A*, then the right and left ϑ-palindromic closures of any factor of a ϑ-standard word are also factors of some ϑ-standard word. We also introduce the class of pseudostandard words with “seed”, obtained by iterated pseudopalindrome closure starting...
On some properties of doubly-periodic words
We study the functional equation: where and are words over an alphabet . In particular we prove a «structure result» for the inner factors : for suitably chosen words one has: ,
On subgraphs without large components
We consider, for a positive integer , induced subgraphs in which each component has order at most . Such a subgraph is said to be -divided. We show that finding large induced subgraphs with this property is NP-complete. We also consider a related graph-coloring problem: how many colors are required in a vertex coloring in which each color class induces a -divided subgraph. We show that the problem of determining whether some given number of colors suffice is NP-complete, even for -coloring...
On substitution invariant Sturmian words: an application of Rauzy fractals
Sturmian words are infinite words that have exactly n+1 factors of length n for every positive integer n. A Sturmian word sα,p is also defined as a coding over a two-letter alphabet of the orbit of point ρ under the action of the irrational rotation Rα : x → x + α (mod 1). A substitution fixes a Sturmian word if and only if it is invertible. The main object of the present paper is to investigate Rauzy fractals associated with two-letter invertible substitutions. As an application, we give...
On synchronized sequences and their separators
We introduce the notion of a -synchronized sequence, where is an integer larger than 1. Roughly speaking, a sequence of natural numbers is said to be -synchronized if its graph is represented, in base , by a right synchronized rational relation. This is an intermediate notion between -automatic and -regular sequences. Indeed, we show that the class of -automatic sequences is equal to the class of bounded -synchronized sequences and that the class of -synchronized sequences is strictly...
On synchronized sequences and their separators
We introduce the notion of a k-synchronized sequence, where k is an integer larger than 1. Roughly speaking, a sequence of natural numbers is said to be k-synchronized if its graph is represented, in base k, by a right synchronized rational relation. This is an intermediate notion between k-automatic and k-regular sequences. Indeed, we show that the class of k-automatic sequences is equal to the class of bounded k-synchronized sequences and that the class of k-synchronized sequences is...
On ternary square-free circular words.
On the approximation of min split-coloring and min cocoloring.
On the binary expansion of irrational algebraic numbers
On the complexity of covering nodes by K node-disjoint cycles
On the complexity of infinite sequences. (Sur la complexité des suites infinies.)
On the complexity of the subgraph problem
On the computational complexity of centers locating in a graph
It is shown that the problem of finding a minimum -basis, the -center problem, and the -median problem are -complete even in the case of such communication networks as planar graphs with maximum degree 3. Moreover, a near optimal -center problem is also -complete.
On the computational complexity of (O,P)-partition problems
We prove that for any additive hereditary property P > O, it is NP-hard to decide if a given graph G allows a vertex partition V(G) = A∪B such that G[A] ∈ 𝓞 (i.e., A is independent) and G[B] ∈ P.
On the conjectures of Rauzy and Shallit for infinite words
We show a connection between a recent conjecture of Shallit and an older conjecture of Rauzy for infinite words on a finite alphabet. More precisely we show that a Rauzy-like conjecture is equivalent to Shallit's. In passing we correct a misprint in Rauzy's conjecture.
On the D0L Repetition Threshold
The repetition threshold is a measure of the extent to which there need to be consecutive (partial) repetitions of finite words within infinite words over alphabets of various sizes. Dejean's Conjecture, which has been recently proven, provides this threshold for all alphabet sizes. Motivated by a question of Krieger, we deal here with the analogous threshold when the infinite word is restricted to be a D0L word. Our main result is that, asymptotically, this threshold does not exceed the unrestricted...
On the decidability of semigroup freeness∗
This paper deals with the decidability of semigroup freeness. More precisely, the freeness problem over a semigroup S is defined as: given a finite subset X ⊆ S, decide whether each element of S has at most one factorization over X. To date, the decidabilities of the following two freeness problems have been closely examined. In 1953, Sardinas and Patterson proposed a now famous algorithm for the freeness problem over the free monoids....