23,155 research outputs found
Nonlinear Energetic Particle Transport in the Presence of Multiple Alfvenic Waves in ITER
This work presents the results of a multi mode ITER study on Toroidal Alfven
Eigenmodes, using the nonlinear hybrid HAGIS-LIGKA model. It is found that main
conclusions from earlier studies of ASDEX Upgrade discharges can be transferred
to the ITER scenario: global, nonlinear effects are crucial for the evolution
of the multi mode scenario. This work focuses on the ITER 15 MA baseline
scenario with with a safety factor at the magnetic axis of 0.986. The
least damped eigenmodes of the system are identified with the gyrokinetic,
non-perturbative LIGKA solver, concerning mode structure, frequency and
damping. Taking into account all weakly damped modes that can be identified
linearly, nonlinear simulations with HAGIS reveal strong multi mode behavior:
while in some parameter range, quasi-linear estimates turn out to be reasonable
approximations for the nonlinearly relaxed energetic particle profile, under
certain conditions low-n TAE branches can be excited. As a consequence, not
only grow amplitudes of all modes to (up to orders of magnitude) higher values
compared to the single mode cases but also, strong redistribution is triggered
in the outer radial area between 0.6 and 0.85, far above
quasi-linear estimates.Comment: 14 pages, 20 figures; To be published as special issue in PPCF
12/2015 for EPS Lisbon invited tal
A radial analogue of Poisson's summation formula with applications to powder diffraction and pinwheel patterns
Diffraction images with continuous rotation symmetry arise from amorphous
systems, but also from regular crystals when investigated by powder
diffraction. On the theoretical side, pinwheel patterns and their higher
dimensional generalisations display such symmetries as well, in spite of being
perfectly ordered. We present first steps and results towards a general frame
to investigate such systems, with emphasis on statistical properties that are
helpful to understand and compare the diffraction images. An alternative
substitution rule for the pinwheel tiling, with two different prototiles,
permits the derivation of several combinatorial and spectral properties of this
still somewhat enigmatic example. These results are compared with properties of
the square lattice and its powder diffraction.Comment: 16 pages, 8 figure
Domino: exploring mobile collaborative software adaptation
Social Proximity Applications (SPAs) are a promising new area for ubicomp software that exploits the everyday changes in the proximity of mobile users. While a number of applications facilitate simple file sharing between coâpresent users, this paper explores opportunities for recommending and sharing software between users. We describe an architecture that allows the recommendation of new system components from systems with similar histories of use. Software components and usage histories are exchanged between mobile users who are in proximity with each other. We apply this architecture in a mobile strategy game in which players adapt and upgrade their game using components from other players, progressing through the game through sharing tools and history. More broadly, we discuss the general application of this technique as well as the security and privacy challenges to such an approach
Enumeration of Matchings: Problems and Progress
This document is built around a list of thirty-two problems in enumeration of
matchings, the first twenty of which were presented in a lecture at MSRI in the
fall of 1996. I begin with a capsule history of the topic of enumeration of
matchings. The twenty original problems, with commentary, comprise the bulk of
the article. I give an account of the progress that has been made on these
problems as of this writing, and include pointers to both the printed and
on-line literature; roughly half of the original twenty problems were solved by
participants in the MSRI Workshop on Combinatorics, their students, and others,
between 1996 and 1999. The article concludes with a dozen new open problems.
(Note: This article supersedes math.CO/9801060 and math.CO/9801061.)Comment: 1+37 pages; to appear in "New Perspectives in Geometric
Combinatorics" (ed. by Billera, Bjorner, Green, Simeon, and Stanley),
Mathematical Science Research Institute publication #37, Cambridge University
Press, 199
Quasiperiodicity and non-computability in tilings
We study tilings of the plane that combine strong properties of different
nature: combinatorial and algorithmic. We prove existence of a tile set that
accepts only quasiperiodic and non-recursive tilings. Our construction is based
on the fixed point construction; we improve this general technique and make it
enforce the property of local regularity of tilings needed for
quasiperiodicity. We prove also a stronger result: any effectively closed set
can be recursively transformed into a tile set so that the Turing degrees of
the resulted tilings consists exactly of the upper cone based on the Turing
degrees of the later.Comment: v3: the version accepted to MFCS 201
Infinite games with finite knowledge gaps
Infinite games where several players seek to coordinate under imperfect
information are deemed to be undecidable, unless the information is
hierarchically ordered among the players.
We identify a class of games for which joint winning strategies can be
constructed effectively without restricting the direction of information flow.
Instead, our condition requires that the players attain common knowledge about
the actual state of the game over and over again along every play.
We show that it is decidable whether a given game satisfies the condition,
and prove tight complexity bounds for the strategy synthesis problem under
-regular winning conditions given by parity automata.Comment: 39 pages; 2nd revision; submitted to Information and Computatio
Some remarks on sign-balanced and maj-balanced posets
Let P be a poset with elements 1,2,...,n. We say that P is sign-balanced if
exactly half the linear extensions of P (regarded as permutations of 1,2,...,n)
are even permutations, i.e., have an even number of inversions. This concept
first arose in the work of Frank Ruskey, who was interested in the efficient
generation of all linear extensions of P. We survey a number of techniques for
showing that posets are sign-balanced, and more generally, computing their
"imbalance." There are close connections with domino tilings and, for certain
posets, a "domino generalization" of Schur functions due to Carre and Leclerc.
We also say that P is maj-balanced if exactly half the linear extensions of P
have even major index. We discuss some similarities and some differences
between sign-balanced and maj-balanced posets.Comment: 30 pages. Some inaccuracies in Section 3 have been corrected, and
Conjecture 3.6 has been adde
Subshifts as Models for MSO Logic
We study the Monadic Second Order (MSO) Hierarchy over colourings of the
discrete plane, and draw links between classes of formula and classes of
subshifts. We give a characterization of existential MSO in terms of
projections of tilings, and of universal sentences in terms of combinations of
"pattern counting" subshifts. Conversely, we characterise logic fragments
corresponding to various classes of subshifts (subshifts of finite type, sofic
subshifts, all subshifts). Finally, we show by a separation result how the
situation here is different from the case of tiling pictures studied earlier by
Giammarresi et al.Comment: arXiv admin note: substantial text overlap with arXiv:0904.245
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