14,243 research outputs found
The Lazy Flipper: MAP Inference in Higher-Order Graphical Models by Depth-limited Exhaustive Search
This article presents a new search algorithm for the NP-hard problem of
optimizing functions of binary variables that decompose according to a
graphical model. It can be applied to models of any order and structure. The
main novelty is a technique to constrain the search space based on the topology
of the model. When pursued to the full search depth, the algorithm is
guaranteed to converge to a global optimum, passing through a series of
monotonously improving local optima that are guaranteed to be optimal within a
given and increasing Hamming distance. For a search depth of 1, it specializes
to Iterated Conditional Modes. Between these extremes, a useful tradeoff
between approximation quality and runtime is established. Experiments on models
derived from both illustrative and real problems show that approximations found
with limited search depth match or improve those obtained by state-of-the-art
methods based on message passing and linear programming.Comment: C++ Source Code available from
http://hci.iwr.uni-heidelberg.de/software.ph
On the probabilistic min spanning tree Problem
We study a probabilistic optimization model for min spanning tree, where any vertex vi of the input-graph G(V,E) has some presence probability pi in the final instance GâČ â G that will effectively be optimized. Suppose that when this ârealâ instance GâČ becomes known, a spanning tree T, called anticipatory or a priori spanning tree, has already been computed in G and one can run a quick algorithm (quicker than one that recomputes from scratch), called modification strategy, that modifies the anticipatory tree T in order to fit G âČ. The goal is to compute an anticipatory spanning tree of G such that, its modification for any G âČ â G is optimal for G âČ. This is what we call probabilistic min spanning tree problem. In this paper we study complexity and approximation of probabilistic min spanning tree in complete graphs under two distinct modification strategies leading to different complexity results for the problem. For the first of the strategies developed, we also study two natural subproblems of probabilistic min spanning tree, namely, the probabilistic metric min spanning tree and the probabilistic min spanning tree 1,2 that deal with metric complete graphs and complete graphs with edge-weights either 1, or 2, respectively
A fast ILP-based Heuristic for the robust design of Body Wireless Sensor Networks
We consider the problem of optimally designing a body wireless sensor
network, while taking into account the uncertainty of data generation of
biosensors. Since the related min-max robustness Integer Linear Programming
(ILP) problem can be difficult to solve even for state-of-the-art commercial
optimization solvers, we propose an original heuristic for its solution. The
heuristic combines deterministic and probabilistic variable fixing strategies,
guided by the information coming from strengthened linear relaxations of the
ILP robust model, and includes a very large neighborhood search for reparation
and improvement of generated solutions, formulated as an ILP problem solved
exactly. Computational tests on realistic instances show that our heuristic
finds solutions of much higher quality than a state-of-the-art solver and than
an effective benchmark heuristic.Comment: This is the authors' final version of the paper published in G.
Squillero and K. Sim (Eds.): EvoApplications 2017, Part I, LNCS 10199, pp.
1-17, 2017. DOI: 10.1007/978-3-319-55849-3\_16. The final publication is
available at Springer via http://dx.doi.org/10.1007/978-3-319-55849-3_1
Towards three-loop QCD corrections to the time-like splitting functions
We report on the status of a direct computation of the time-like splitting
functions at next-to-next-to-leading order in QCD. Time-like splitting
functions govern the collinear kinematics of inclusive hadron production and
the evolution of the parton fragmentation distributions. Current knowledge
about them at three loops has been inferred by means of crossing symmetry from
their related space-like counterparts, which has left certain parts of the
off-diagonal quark-gluon splitting function undetermined. This motivates an
independent calculation from first principles. We review the tools and methods
which are applied to attack the problem.Comment: 11 pages, 5 figures; presented at the Epiphany Conference 2015
(Cracow, Poland); additional files: MassFactorization.n
On unique continuation for solutions of the Schr{\"o}dinger equation on trees
We prove that if a solution of the time-dependent Schr{\"o}dinger equation on
an homogeneous tree with bounded potential decays fast at two distinct times
then the solution is trivial. For the free Schr{\"o}dinger operator, we use the
spectral theory of the Laplacian and complex analysis and obtain a
characterization of the initial conditions that lead to a sharp decay at any
time. We then use the recent spectral decomposition of the Schr{\"o}dinger
operator with compactly supported potential due to Colin de Verdi{\`e}rre and
Turc to extend our results in the presence of such potentials. Finally, we use
real variable methods first introduced by Escauriaza, Kenig, Ponce and Vega to
establish a general sharp result in the case of bounded potentials
An (MI)LP-based Primal Heuristic for 3-Architecture Connected Facility Location in Urban Access Network Design
We investigate the 3-architecture Connected Facility Location Problem arising
in the design of urban telecommunication access networks. We propose an
original optimization model for the problem that includes additional variables
and constraints to take into account wireless signal coverage. Since the
problem can prove challenging even for modern state-of-the art optimization
solvers, we propose to solve it by an original primal heuristic which combines
a probabilistic fixing procedure, guided by peculiar Linear Programming
relaxations, with an exact MIP heuristic, based on a very large neighborhood
search. Computational experiments on a set of realistic instances show that our
heuristic can find solutions associated with much lower optimality gaps than a
state-of-the-art solver.Comment: This is the authors' final version of the paper published in:
Squillero G., Burelli P. (eds), EvoApplications 2016: Applications of
Evolutionary Computation, LNCS 9597, pp. 283-298, 2016. DOI:
10.1007/978-3-319-31204-0_19. The final publication is available at Springer
via http://dx.doi.org/10.1007/978-3-319-31204-0_1
Physics of the Power Corrections in QCD
We review the physics of the power corrections to the parton model. In the
first part, we consider the power corrections which characterize the infrared
sensitivity of Feynman graphs when the contribution of short distances
dominates. The second part is devoted to the hypothetical power corrections
associated with nonperturbative effects at small distances.Comment: 35 pages, 6 figures. Lecture given by V.I.Zakharov at the Winter
School of physics of ITEP, February 1999. Minor corrections, references adde
Water-soluble SOA from Alkene ozonolysis: composition and droplet activation kinetics inferences from analysis of CCN activity
Cloud formation characteristics of the water-soluble organic fraction (WSOC) of secondary organic aerosol (SOA) formed from the ozonolysis of alkene hydrocarbons (terpinolene, 1-methlycycloheptene and cycloheptene) are studied. Based on size-resolved measurements of CCN activity (of the pure and salted WSOC samples) we estimate the average molar volume and surface tension depression associated with the WSOC using Köhler Theory Analysis (KTA). Consistent with known speciation, the results suggest that the WSOC are composed of low molecular weight species, with an effective molar mass below 200 g mol^(â1). The water-soluble carbon is also surface-active, depressing surface tension 10â15% from that of pure water (at CCN-relevant concentrations). The inherent hygroscopicity parameter, Îș, of the WSOC ranges between 0.17 and 0.25; if surface tension depression and molar volume effects are considered in Îș, a remarkably constant "apparent" hygroscopicity ~0.3 emerges for all samples considered. This implies that the volume fraction of soluble material in the parent aerosol is the key composition parameter required for prediction of the SOA hygroscopicity, as shifts in molar volume across samples are compensated by changes in surface tension. Finally, using "threshold droplet growth analysis", the water-soluble organics in all samples considered do not affect CCN activation kinetics
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