Stochastic Analysis of a Churn-Tolerant Structured Peer-to-Peer Scheme

We present and analyze a simple and general scheme to build a churn (fault)-tolerant structured Peer-to-Peer (P2P) network. Our scheme shows how to "convert" a static network into a dynamic distributed hash table(DHT)-based P2P network such that all the good properties of the static network are guaranteed with high probability (w.h.p). Applying our scheme to a cube-connected cycles network, for example, yields a $O(\log N)$ degree connected network, in which every search succeeds in $O(\log N)$ hops w.h.p., using $O(\log N)$ messages, where $N$ is the expected stable network size. Our scheme has an constant storage overhead (the number of nodes responsible for servicing a data item) and an $O(\log N)$ overhead (messages and time) per insertion and essentially no overhead for deletions. All these bounds are essentially optimal. While DHT schemes with similar guarantees are already known in the literature, this work is new in the following aspects: (1) It presents a rigorous mathematical analysis of the scheme under a general stochastic model of churn and shows the above guarantees; (2) The theoretical analysis is complemented by a simulation-based analysis that validates the asymptotic bounds even in moderately sized networks and also studies performance under changing stable network size; (3) The presented scheme seems especially suitable for maintaining dynamic structures under churn efficiently. In particular, we show that a spanning tree of low diameter can be efficiently maintained in constant time and logarithmic number of messages per insertion or deletion w.h.p. Keywords: P2P Network, DHT Scheme, Churn, Dynamic Spanning Tree, Stochastic Analysis

Placing regenerators in optical networks to satisfy multiple sets of requests

The placement of regenerators in optical networks has become an active area of research during the last years. Given a set of lightpaths in a network G and a positive integer d, regenerators must be placed in such a way that in any lightpath there are no more than d hops without meeting a regenerator. While most of the research has focused on heuristics and simulations, the first theoretical study of the problem has been recently provided in [10], where the considered cost function is the number of locations in the network hosting regenerators. Nevertheless, in many situations a more accurate estimation of the real cost of the network is given by the total number of regenerators placed at the nodes, and this is the cost function we consider. Furthermore, in our model we assume that we are given a finite set of p possible traffic patterns (each given by a set of lightpaths), and our objective is to place the minimum number of regenerators at the nodes so that each of the traffic patterns is satisfied. While this problem can be easily solved when d = 1 or p = 1, we prove that for any fixed d,p ≥ 2 it does not admit a PTASUnknown control sequence '\textsc', even if G has maximum degree at most 3 and the lightpaths have length O(d)(d). We complement this hardness result with a constant-factor approximation algorithm with ratio ln (d ·p). We then study the case where G is a path, proving that the problem is NP-hard for any d,p ≥ 2, even if there are two edges of the path such that any lightpath uses at least one of them. Interestingly, we show that the problem is polynomial-time solvable in paths when all the lightpaths share the first edge of the path, as well as when the number of lightpaths sharing an edge is bounded. Finally, we generalize our model in two natural directions, which allows us to capture the model of [10] as a particular case, and we settle some questions that were left open in [10]

Distance-Aware Selective Online Query Processing Over Large Distributed Graphs

Performing online selective queries against graphs is a challenging problem due to the unbounded nature of graph queries which leads to poor computation locality. It becomes even difficult when a graph is too large to be fit in the memory. Although there have been emerging efforts on managing large graphs in a distributed and parallel setting, e.g., Pregel, HaLoop and etc, these computing frameworks are designed from the perspective of scalability instead of the query efficiency. In this work, we present our solution methodology for online selective graph queries based on the shortest path distance semantic, which finds various applications in practice. The essential intuition is to build a distance-aware index for online distance-based query processing and to eliminate redundant graph traversal as much as possible. We discuss how the solution can be applied to two types of research problems, distance join and vertex set bonding, which are distance-based graph pattern discovery and finding the structure-wise bonding of vertices, respectively

SPIN WAVES IN REENTRANT SPIN GLASS Fe0.9Ti0.55Mg1.55O4

Strong thermoremanent magnetization appears at below 25 K in low magnetic fields. At any temperature below 57 K the high field magnetization satisfies the spin wave theory but the magnetization does not follow the T3/2 law. That suggests a mixed-phase which appears before the strong irreversibility region

Magnetic behaviour of Co-Cr alloys above the critical concentration for ferromagnetism

We have investigated Co-Cr alloys with chromium concentrations slightly larger than the critical concentration for long range magnetic order (25 %). Our results suggest a strong inhomogeneous magnetic state consisting of clusters with a large distribution of formation températures. At low temperatures one observes a superparamagnetic to ferromagnetic transition followed by a ferromagnetic to spin glass transition at lower temperature in the range of concentration 25 % to 29 %.Nous avons étudié des alliages Co-Cr dans un domaine de concentrations en chrome légèrement supérieures à la concentration critique de disparition du ferromagnétisme (25 %). Nos résultats suggèrent un état magnétique très inhomogène, formé d'amas présentant une large distribution de températures de formation. Aux basses températures, on observe une transition superparamagnétisme-ferromagnétisme et une transition ferro-verre de spin à plus basse température dans la gamme de concentration 25 %-29 %

Application de la dilatométrie et des mesures magnétiques à l'étude de la transformation martensitique sous contrainte d'un alliage Fe-20 Ni-0,5 C

We have devised a method for measuring the variations in magnetic properties in the study of martensitic transformation. After calibration we can directly measure the quantity of ferromagnetic martensite formed. This method has been combined with dilatometry in order to study martensitic transformation under stress for different kinds of mechanical tests. We give quantitative examples of the variations of transformation kinetics and measurements of déformation due to transformation plasticity for martensitic transformation of an Fe-20 Ni-0.5 C alloy during anisothermal creep tests and tensile tests.Nous avons mis au point une méthode de suivi des variations de propriétés magnétiques pour l'étude de la transformation martensitique. Après étalonnage cette méthode nous permet d'atteindre directement la quantité de phase martensitique ferromagnétique formée. Nous avons couplé cette méthode à la dilatométrie pour étudier la transformation martensitique sous contrainte pour divers types de sollicitations mécaniques. Nous donnons des exemples quantitatifs des variations de la cinétique de transformation et de la plasticité de transformation pour la transformation martensitique d'un alliage Fe-20 Ni-0,5 C au cours d'essais de fluage anisotherme et d'essais de traction

Small k-Dominating Sets in Planar Graphs with Applications

A subset of nodes S in a graph G is called k-dominating if, for every node u of the graph, the distance from u to S is at most k. We consider the parameter k (G) de ned as the cardinality of the smallest k-dominating set of G. For planar graphs, we show that for every > 0 and for every k > (5=7 + )D, k (G) = O(1=). For several subclasses of planar graphs of diameter D, we show that k (G) is bounded by a constant for k > D=2. We conjecture that the same result holds for every planar graph. This problem is motivated by the design of routing schemes with compact data structures

Acceleration of ions up to 20MeV/nucleon in the ultrashort, high-intensity regime

The measurements reported here provide scaling laws for the ion acceleration process in the regime of ultrashort (50 fs), ultrahigh contrast (1010) and ultrahigh intensity (> 1020W/cm 2), never investigated previously. The scaling of the accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10nm), target material (C and Al) and laser light polar- ization (circular and linear) at 35° and normal laser incidence. A twofold increase in proton energy and an order of magnitude enhancement in ion flux have been observed over the investigated thickness range at 35° angle of incidence. Further- more, at normal laser incidence, measured peak proton energies of about 20 MeV are observed almost independently of the target thickness over a wide range (50nm- 10 μm). 1

Improved Compact Routing Tables for Planar Networks via Orderly Spanning Trees

We address the problem of designing compact routing tables for an unlabeled connected n-node planar network G. For each node r of G, the designer is given a routing spanning tree Tr of G rooted at r, which speci es the routes for sending packets from r to the rest of G