Finding Paths in Grids with Forbidden Transitions

Une transition dans un graphe est une paire d'arêtes incidente a un même sommet. Etant donnés un graphe G = (V, E), deux sommets s, t ∈ V et un ensemble associé de transitions interdites F ⊆ E × E, le problème de chemin évitant des transitions interdites consiste à décider s’il existe un chemin élémentaire de s à t qui n’utilise aucune des transitions de F. C’est-à-dire qu’il est interdit d’emprunter consécutivement deux arêtes qui soient une paire de F. Ce problème est motivé par le routage dans les réseaux routiers (où une transition interdite représente une interdiction de tourner) ainsi que dans les réseaux optiques avec des noeuds asymétriques. Nous prouvons que le problème est NP-difficile dans les graphes planaires et plus particulièrement dans les grilles. Nous montrons également que le problème peut être résolu en temps polynomial dans la classe des graphes de largeur arborescente bornée

Finding Paths in Grids with Forbidden Transitions

Une transition dans un graphe est une paire d'arêtes incidente a un même sommet. Etant donnés un graphe G = (V, E), deux sommets s, t ∈ V et un ensemble associé de transitions interdites F ⊆ E × E, le problème de chemin évitant des transitions interdites consiste à décider s’il existe un chemin élémentaire de s à t qui n’utilise aucune des transitions de F. C’est-à-dire qu’il est interdit d’emprunter consécutivement deux arêtes qui soient une paire de F. Ce problème est motivé par le routage dans les réseaux routiers (où une transition interdite représente une interdiction de tourner) ainsi que dans les réseaux optiques avec des noeuds asymétriques. Nous prouvons que le problème est NP-difficile dans les graphes planaires et plus particulièrement dans les grilles. Nous montrons également que le problème peut être résolu en temps polynomial dans la classe des graphes de largeur arborescente bornée

Shortest path and maximum flow problems in planar flow networks with additive gains and losses

In contrast to traditional flow networks, in additive flow networks, to every edge e is assigned a gain factor g(e) which represents the loss or gain of the flow while using edge e. Hence, if a flow f(e) enters the edge e and f(e) is less than the designated capacity of e, then f(e) + g(e) = 0 units of flow reach the end point of e, provided e is used, i.e., provided f(e) != 0. In this report we study the maximum flow problem in additive flow networks, which we prove to be NP-hard even when the underlying graphs of additive flow networks are planar. We also investigate the shortest path problem, when to every edge e is assigned a cost value for every unit flow entering edge e, which we show to be NP-hard in the strong sense even when the additive flow networks are planar

The Minimum Shared Edges Problem on Grid-like Graphs

We study the NP-hard Minimum Shared Edges (MSE) problem on graphs: decide whether it is possible to route $p$ paths from a start vertex to a target vertex in a given graph while using at most $k$ edges more than once. We show that MSE can be decided on bounded (i.e. finite) grids in linear time when both dimensions are either small or large compared to the number $p$ of paths. On the contrary, we show that MSE remains NP-hard on subgraphs of bounded grids. Finally, we study MSE from a parametrised complexity point of view. It is known that MSE is fixed-parameter tractable with respect to the number $p$ of paths. We show that, under standard complexity-theoretical assumptions, the problem parametrised by the combined parameter $k$, $p$, maximum degree, diameter, and treewidth does not admit a polynomial-size problem kernel, even when restricted to planar graphs

Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars

It is generally accepted that the atmospheres of cool/lukewarm stars of spectral types A and later are described well by LTE model atmospheres, while the O-type stars require a detailed treatment of NLTE effects. Here model atmosphere structures, spectral energy distributions and synthetic spectra computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE are compared. Their ability to reproduce observations for effective temperatures between 15000 and 35000 K are verified. Strengths and weaknesses of the different approaches are identified. Recommendations are made as to how to improve the models in order to derive unbiased stellar parameters and chemical abundances in future applications, with special emphasis on Gaia science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics: Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er

Rapid Tunneling and Percolation in the Landscape

Motivated by the possibility of a string landscape, we reexamine tunneling of a scalar field across single/multiple barriers. Recent investigations have suggested modifications to the usual picture of false vacuum decay that lead to efficient and rapid tunneling in the landscape when certain conditions are met. This can be due to stringy effects (e.g. tunneling via the DBI action), or by effects arising due to the presence of multiple vacua (e.g. resonance tunneling). In this paper we discuss both DBI tunneling and resonance tunneling. We provide a QFT treatment of resonance tunneling using the Schr\"odinger functional approach. We also show how DBI tunneling for supercritical barriers can naturally lead to conditions suitable for resonance tunneling. We argue using basic ideas from percolation theory that tunneling can be rapid in a landscape where a typical vacuum has multiple decay channels and discuss various cosmological implications. This rapidity vacuum decay can happen even if there are no resonance/DBI tunneling enhancements, solely due to the presence of a large number of decay channels. Finally, we consider various ways of circumventing a recent no-go theorem for resonance tunneling in quantum field theory.Comment: 47 pages, 16 figures. Acknowledgements adde

The Complexity of Connectivity Problems in Forbidden-Transition Graphs And Edge-Colored Graphs

The notion of forbidden-transition graphs allows for a robust generalization of walks in graphs. In a forbidden-transition graph, every pair of edges incident to a common vertex is permitted or forbidden; a walk is compatible if all pairs of consecutive edges on the walk are permitted. Forbidden-transition graphs and related models have found applications in a variety of fields, such as routing in optical telecommunication networks, road networks, and bio-informatics. We initiate the study of fundamental connectivity problems from the point of view of parameterized complexity, including an in-depth study of tractability with regards to various graph-width parameters. Among several results, we prove that finding a simple compatible path between given endpoints in a forbidden-transition graph is W[1]-hard when parameterized by the vertex-deletion distance to a linear forest (so it is also hard when parameterized by pathwidth or treewidth). On the other hand, we show an algebraic trick that yields tractability when parameterized by treewidth of finding a properly colored Hamiltonian cycle in an edge-colored graph; properly colored walks in edge-colored graphs is one of the most studied special cases of compatible walks in forbidden-transition graphs