On vertex neighborhood in minimal imperfect graphs

AbstractLubiw (J. Combin. Theory Ser. B 51 (1991) 24) conjectures that in a minimal imperfect Berge graph, the neighborhood graph N(v) of any vertex v must be connected; this conjecture implies a well known Chvátal's conjecture (Chvátal, First Workshop on Perfect Graphs, Princeton, 1993) which states that N(v) must contain a P4. In this note we will prove an intermediary conjecture for some classes of minimal imperfect graphs. It is well known that a graph is P4-free if, and only if, every induced subgraph with at least two vertices either is disconnected or its complement is disconnected; this characterization implies that P4-free graphs can be constructed by complete join and disjoint union from isolated vertices. We propose to replace P4-free graphs by a similar construction using bipartite graphs instead of isolated vertices

Anisotropic long-range interaction investigated with cold atoms

In two dimensions, a system of self-gravitating particles collapses and forms a singularity in finite time below a critical temperature $T_c$. We investigate experimentally a quasi two-dimensional cloud of cold neutral atoms in interaction with two pairs of perpendicular counter-propagating quasi-resonant laser beams, in order to look for a signature of this ideal phase transition: indeed, the radiation pressure forces exerted by the laser beams can be viewed as an anisotropic, and non-potential, generalization of two-dimensional self-gravity. We first show that our experiment operates in a parameter range which should be suitable to observe the collapse transition. However, the experiment unveils only a moderate compression instead of a phase transition between the two phases. A three-dimensional numerical simulation shows that both the finite small thickness of the cloud, which induces a competition between the effective gravity force and the repulsive force due to multiple scattering, and the atomic losses due to heating in the third dimension, contribute to smearing the transition.Comment: 11 pages, 8 figures, accepted in Physical Review

Former à la classe inversée en contexte de visioconférence : retours sur deux itérations d'une recherche orientée par la conception

National audienceAs part of a collaborative design-based research project, flipped classroom training in a videoconferencing context was given twice to teachers. The pedagogical design of this training was structured in a framework based on the creation of a learning community, the learning presence and the real experience as a learner of a flipped classroom experience in a videoconferencing context. Following the first iteration, empirical findings, from the point of view of learning, as well as the activity of the instructors or the technopedagogical features implemented, led to the development of a first research project aimed at describing the presence created within this community, according to the learning presence model [11]. This work helped to highlight a certain lack of socio-cognitive presence and to raise questions about the pedagogical presence.Dans notre projet de recherche collaborative orientée par la conception, une formation sur la classe inversée en contexte de visioconférence a été donnée à deux reprises à des enseignants et conseillers pédagogiques. Le design pédagogique de cette formation se structurait dans un cadre misant sur la création d'une communauté d'apprentissage, la présence à distance et le vécu en tant qu'apprenant d'une expérience de classe inversée en contexte de visioconférence. Suite à la première itération, des constats empiriques, tant du point de vue de l'apprentissage, que de l'activité des formateurs ou des caractéristiques technopédagogiques mises en oeuvre, ont motivé l'élaboration d'un premier travail de recherche visant à décrire la présence suscitée au sein de cette communauté, selon le modèle de la présence à distance [11]. Ce travail a notamment permis de mettre en évidence une certaine carence en matière de présence sociocognitive et de soulever des questions sur la présence pédagogique

The CD85j+ NK Cell Subset Potently Controls HIV-1 Replication in Autologous Dendritic Cells

Natural killer (NK) cells and dendritic cells (DC) are thought to play critical roles in the first phases of HIV infection. In this study, we examined changes in the NK cell repertoire and functions occurring in response to early interaction with HIV-infected DC, using an autologous in vitro NK/DC coculture system. We show that NK cell interaction with HIV-1-infected autologous monocyte-derived DC (MDDC) modulates NK receptor expression. In particular, expression of the CD85j receptor on NK cells was strongly down-regulated upon coculture with HIV-1-infected MDDC. We demonstrate that CD85j+ NK cells exert potent control of HIV-1 replication in single-round and productively HIV-1-infected MDDC, whereas CD85j− NK cells induce a modest and transient decrease of HIV-1 replication. HIV-1 suppression in MDCC by CD85j+ NK cells required cell-to-cell contact and did not appear mediated by cytotoxicity or by soluble factors. HIV-1 inhibition was abolished when NK-MDDC interaction through the CD85j receptor was blocked with a recombinant CD85j molecule, whereas inhibition was only slightly counteracted by blocking HLA class I molecules, which are known CD85j ligands. After masking HLA class I molecules with specific antibodies, a fraction of HIV-1 infected MDDC was still strongly stained by a recombinant CD85j protein. These results suggest that CD85j+ NK cell inhibition of HIV-1 replication in MDDC is mainly mediated by CD85j interaction with an unknown ligand (distinct from HLA class I molecules) preferentially expressed on HIV-1-infected MDDC

A Generalization of Slightly Triangulated Graphs

... In this paper, we introduce a wider class of graphs, obtained by relaxing the neighbourhood condition. We also provide polynomial algorithms to recognize and to produce an optimal coloring of a subclass of slightly triangulated graphs

On Minimal Imperfect Graphs without Induced P5

In this paper, we summarize previous known results on P5-free minimal imperfect graphs (i.e. minimal imperfect graphs not containing a path on 5 vertices as induced subgraph) and we introduce two new classes of graphs (defined by a local property) that contain P5-free graphs. Next, we show that most of the results concerning P5-free graphs can be extended to these classes. Moreover, we present a structural characterization of these graphs which leads to some new results. In particular, we prove that the Strong Perfect Graph Conjecture holds true for P5-free and F-free graphs where F is any connected congfiuration on 5 vertices not containing an induced 2K 2 . 1 Introduction A graph is perfect if the vertices of any induced subgraph H can be colored, in such a way that no two adjacent vertices receive the same color, with a number of colors (denoted by (H)) not exceeding the cardinality !(H) of a maximum clique of H. A graph is minimal imperfect if all its proper induced sub..

Biopolymers as Encapsulating Agents for the Immobilization of Prussian Blue and Analogues for the Sorption of Cesium

International audiencePrussian Blue (PB, iron hexacyanoferrate) is an efficient micron-sized sorbent that can be encapsulated in a biopolymer matrix (chitin) for the treatment of Cs (I)-bearing solutions. Different conditionings have been designed for the treatment of Cs-contaminated effluents (with natural Cs (I) solutions, and with 137Cs spiked solutions). The sorbents conditioned under the form of beads or discs can be used in columns while sponges can be used for the treatment of accidental discharge of effluents (contaminated water absorption by the sponge, metal ions sorption on the ion-exchanger, treated water removal by centrifugation, metal concentration in the composite material)

On minimal cutsets in P5-free minimal imperfect graphs

International audienc