10 research outputs found
Expanding graphs, Ramanujan graphs, and 1-factor perturbations
We construct (k+-1)-regular graphs which provide sequences of expanders by
adding or substracting appropriate 1-factors from given sequences of k-regular
graphs. We compute numerical examples in a few cases for which the given
sequences are from the work of Lubotzky, Phillips, and Sarnak (with k-1 the
order of a finite field). If k+1 = 7, our construction results in a sequence of
7-regular expanders with all spectral gaps at least about 1.52
Competitive ratio of List Scheduling on uniform machines andrandomized heuristics
We study online scheduling on m uniform machines, where mâ1 of them have a reference speed 1 and the last one a speed s with 0â€sâ€1. The competitive ratio of the well-known List Scheduling (LS) algorithm is determined. For some values of s and m=3, LS is proven to be the best deterministic algorithm. We describe a randomized heuristic for m machines. Finally, for the case m=3, we develop and analyze the competitive ratio of a randomized algorithm which outperforms LS for any
On matrices with the Edmonds-Johnson property arising from bidirected graphs
In this paper we study totally half-modular matrices obtained from {0,±1}-matrices with at most two nonzero entries per column by multiplying by 2 some of the columns. We give an excluded-minor characterization of the matrices in this class having strong Chv`atal rank 1. Our result is a special case of a conjecture by Gerards and Schrijver [11]. It also extends a well known theorem of Edmonds and Johnson [10]
New polynomial-time algorithms for Camion bases
Let M be a finite set of vectors in R n of cardinality m and H(M) ={{x â R n: c T x = 0}:c â M} the central hyperplane arrangement represented by M. An independent subset of M of cardinality n is called a Camion basis, if it determines a simplex region in the arrangement H(M). In this paper, we first present a new characterization of Camion bases, in the case where M is the column set of the node-edge incidence matrix (without one row) of a given connected digraph. Then, a general characterization of Camion bases as well as a recognition procedure which runs in O(n 2 m) are given. Finally, an algorithm which finds a Camion basis is presented. For certain classes of matrices, including totally unimodular matrices, it is proven to run in polynomial time and faster than the algorithm due to Fonlupt and Raco
Gallomanie et gallophobie
Au cours de la phase de Nation building qui dĂ©finit lâEurope au XIXe siĂšcle, lâimage de la France a eu un rĂŽle particulier : modĂšle ou contre-modĂšle de civilisation, exemple de nationalisme littĂ©raire et politique Ă imiter ou Ă combattre, vivier de stĂ©rĂ©otypes nationaux dĂ©clinĂ©s dans lâEurope entiĂšre. Pour construire leur propre identitĂ©, les diffĂ©rents pays et les diffĂ©rentes rĂ©gions se sont Ă un moment ou Ă un autre interrogĂ©s sur le mythe français et ont pris position pour assoir leur lĂ©gitimitĂ© ou mieux cerner leur diffĂ©rence. Le volume rĂ©unit dix-huit articles rĂ©digĂ©s par des spĂ©cialistes de diffĂ©rentes disciplines (histoire, littĂ©rature française et Ă©trangĂšre, sciences politiques), en provenance de toute lâEurope, qui Ă©tudient et analysent les reprĂ©sentations de la France Ă lâĂ©tranger. Les auteurs se sont penchĂ©s sur les diffĂ©rentes manifestations de sentiments pro-français et anti-français dans les textes littĂ©raires et politiques, lâiconographie, les manuels dâhistoire et dâenseignement, les articles de journaux, les mĂ©moires, les dictionnaires, afin dâapporter une nouvelle pierre Ă lâhistoire des reprĂ©sentations nationales. Les articles qui composent le volume analysent chacun une facette du « mythe français » et montrent la circulation des mĂȘmes thĂšmes dans diffĂ©rentes aires culturelles europĂ©ennes ; ils donnent ainsi un aperçu reprĂ©sentatif des usages en vigueur et des rĂ©flexions individuelles qui ont alimentĂ© le dĂ©bat international sur lâĂ©laboration des identitĂ©s nationales