Time-dependent optical spectroscopy of GRB 010222: Clues to the gamma-ray burst environment

We present sequential optical spectra of the afterglow of GRB 010222 obtained 1 day apart using the Low-Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck Telescopes. Three low-ionization absorption systems are spectroscopically identified at z 1 = 1.47688, z2 = 1.15628, and z3 = 0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z1a = 1.47590 and z1b = 1.47688. We interpret the z1b = 1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best-fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H2. If the gamma-ray burst took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high-density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed nH I = 102 cm -3. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then nH I ≥ 2 × 104 cm-3 is probably ruled out in the immediate vicinity of the burst

Faster Dynamic Algorithms for Chordal Graphs, and an Application to Phylogeny

International audienceWe improve the current complexities for maintaining a chordal graph by starting with an empty graph and repeatedly adding or deleting edges

Very Fast Instances for Concept Generation

International audienceComputing the maximal bicliques of a bipartite graph is equivalent to generating the concepts of the binary relation defined by the matrix of this graph. We study this problem for special classes of input relations for which concepts can be generated much more efficiently than in the general case; in some special cases, we can even say that the number of concepts is polynomially bounded, and all concepts can be generated particularly quickly

Intersection Dimension of Bipartite Graphs

We introduce a concept of intersection dimension of a graphwith respect to a graph class. This generalizes Ferrers dimension, boxicity, and poset dimension, and leads to interesting new problems. We focus in particular on bipartite graph classes defined as intersection graphs of two kinds of geometric objects. We relate well-known graph classes such as interval bigraphs, two-directional orthogonal ray graphs, chain graphs, and (unit) grid intersection graphs with respect to these dimensions. As an application of these graphtheoretic results, we show that the recognition problems for certain graph classes are NP-complete.Theory and Applications of Models of Computation, 11th Annual Conference, TAMC 2014, Chennai, India, April 11-13, 2014. Proceeding

Développement durable et participation: la démocratie introuvable

Le concept de « participation » est fortement lié à celui de développement durable. Une analyse en termes d'histoire des idées permet de comprendre les différences entre « écologie politique » et « développement durable ». Cette distinction éclaire en retour les différences entre démocratie directe, représentative et participative. La démocratie participative doit être comprise comme une simple modalité de la démocratie représentative et ne constitue pas en soi un facteur de démocratisation. L'article conclut en montrant les affinités entre les idées de « participation » et de « développement durable »

Linear-Time Counting Algorithms for Independent Sets in Chordal Graphs

We study some counting and enumeration problems for chordal graphs, especially concerning independent sets. We first provide the following efficient algorithms for a chordal graph: (1) a linear-time algorithm for counting the number of independent sets; (2) a linear-time algorithm for counting the number of maximum independent sets; (3) a polynomial-time algorithm for counting the number of independent sets of a fixed size. With similar ideas, we show that enumeration (namely, listing) of the independent sets, the maximum independent sets, and the independent sets of a fixed size in a chordal graph can be done in constant amortized time per output. On the other hand, we prove that the following problems for a chordal graph are #P-complete: (1) counting the number of maximal independent sets; (2) counting the number of minimum maximal independent sets. With similar ideas, we also show that finding a minimum weighted maximal independent set in a chordal graph is NP-hard, and even hard to approximate