213 research outputs found
Symblicit algorithms for optimal strategy synthesis in monotonic Markov decision processes
When treating Markov decision processes (MDPs) with large state spaces, using
explicit representations quickly becomes unfeasible. Lately, Wimmer et al. have
proposed a so-called symblicit algorithm for the synthesis of optimal
strategies in MDPs, in the quantitative setting of expected mean-payoff. This
algorithm, based on the strategy iteration algorithm of Howard and Veinott,
efficiently combines symbolic and explicit data structures, and uses binary
decision diagrams as symbolic representation. The aim of this paper is to show
that the new data structure of pseudo-antichains (an extension of antichains)
provides another interesting alternative, especially for the class of monotonic
MDPs. We design efficient pseudo-antichain based symblicit algorithms (with
open source implementations) for two quantitative settings: the expected
mean-payoff and the stochastic shortest path. For two practical applications
coming from automated planning and LTL synthesis, we report promising
experimental results w.r.t. both the run time and the memory consumption.Comment: In Proceedings SYNT 2014, arXiv:1407.493
Softening and Yielding of Soft Glassy Materials
Solids deform and fluids flow, but soft glassy materials, such as emulsions,
foams, suspensions, and pastes, exhibit an intricate mix of solid and
liquid-like behavior. While much progress has been made to understand their
elastic (small strain) and flow (infinite strain) properties, such
understanding is lacking for the softening and yielding phenomena that connect
these asymptotic regimes. Here we present a comprehensive framework for
softening and yielding of soft glassy materials, based on extensive numerical
simulations of oscillatory rheological tests, and show that two distinct
scenarios unfold depending on the material's packing density. For dense
systems, there is a single, pressure-independent strain where the elastic
modulus drops and the particle motion becomes diffusive. In contrast, for
weakly jammed systems, a two-step process arises: at an intermediate softening
strain, the elastic and loss moduli both drop down and then reach a new plateau
value, whereas the particle motion becomes diffusive at the distinctly larger
yield strain. We show that softening is associated with an extensive number of
microscopic contact changes leading to a non-analytic rheological signature.
Moreover, the scaling of the softening strain with pressure suggest the
existence of a novel pressure scale above which softening and yielding
coincide, and we verify the existence of this crossover scale numerically. Our
findings thus evidence the existence of two distinct classes of soft glassy
materials -- jamming dominated and dense -- and show how these can be
distinguished by their rheological fingerprint.Comment: 9 pages, 11 figures, to appear in Soft Matte
Finite-Size Scaling at the Jamming Transition
We present an analysis of finite-size effects in jammed packings of N soft,
frictionless spheres at zero temperature. There is a 1/N correction to the
discrete jump in the contact number at the transition so that jammed packings
exist only above isostaticity. As a result, the canonical power-law scalings of
the contact number and elastic moduli break down at low pressure. These
quantities exhibit scaling collapse with a non-trivial scaling function,
demonstrating that the jamming transition can be considered a phase transition.
Scaling is achieved as a function of N in both 2 and 3 dimensions, indicating
an upper critical dimension of 2.Comment: 5 pages, 3 figure
Soft Sphere Packings at Finite Pressure but Unstable to Shear
When are athermal soft sphere packings jammed ? Any experimentally relevant
definition must at the very least require a jammed packing to resist shear. We
demonstrate that widely used (numerical) protocols in which particles are
compressed together, can and do produce packings which are unstable to shear -
and that the probability of generating such packings reaches one near jamming.
We introduce a new protocol that, by allowing the system to explore different
box shapes as it equilibrates, generates truly jammed packings with strictly
positive shear moduli G. For these packings, the scaling of the average of G is
consistent with earlier results, while the probability distribution P(G)
exhibits novel and rich scalingComment: 5 pages, 6 figures. Resubmitted to Physical Review Letters after a
few change
Contact Changes near Jamming
We probe the onset and effect of contact changes in soft harmonic particle
packings which are sheared quasistatically. We find that the first contact
changes are the creation or breaking of contacts on a single particle. We
characterize the critical strain, statistics of breaking versus making a
contact, and ratio of shear modulus before and after such events, and explain
their finite size scaling relations. For large systems at finite pressure, the
critical strain vanishes but the ratio of shear modulus before and after a
contact change approaches one: linear response remains relevant in large
systems. For finite systems close to jamming the critical strain also vanishes,
but here linear response already breaks down after a single contact change.Comment: 5 pages, 4 figure
Jamming in finite systems: stability, anisotropy, fluctuations and scaling
Athermal packings of soft repulsive spheres exhibit a sharp jamming
transition in the thermodynamic limit. Upon further compression, various
structural and mechanical properties display clean power-law behavior over many
decades in pressure. As with any phase transition, the rounding of such
behavior in finite systems close to the transition plays an important role in
understanding the nature of the transition itself. The situation for jamming is
surprisingly rich: the assumption that jammed packings are isotropic is only
strictly true in the large-size limit, and finite-size has a profound effect on
the very meaning of jamming. Here, we provide a comprehensive numerical study
of finite-size effects in sphere packings above the jamming transition,
focusing on stability as well as the scaling of the contact number and the
elastic response.Comment: 20 pages, 12 figure
Marisa Rey-Henningsen, The World of the Ploughwoman. Folklore and Reality in Matriarchal Northwest Spain
L’Académie des sciences de Finlande vient de publier un ouvrage qui apporte des éléments de réflexion intéressants sur la question du matriarcat qui aurait été naguère pratiqué par quelques sociétés européennes. En se fondant sur une double démarche, de folkloriste et d’anthropologue, l’auteur, Marisa Rey-Henningsen, s’est attachée à décrire et interpréter les rapports entre sexes qui étaient la norme voici encore très peu de temps en Galice. Une structure sociale particulière – le droit de ..
Mesoscopic structure of dry-pressed clay samples from small-angle X-ray scattering measurements . In : Proceedings of the XIIIth International Conference on Small-Angle Scattering
Weakly hydrated samples of platelet-shaped nano-particles obtained by dry-pressing suspensions of the synthetic Na fluorohectorite clay are studied. The particles consist of stacks of several tens of 1 nm-thick nanosilicate platelets. They form a compound of quasi-two-dimensional particles whose average director is aligned with the direction of the uniaxial stress applied at dehydration. Small-angle X-ray scattering images from these samples are either isotropic or anisotropic, depending on the sample orientation with respect to the X-ray beam. From anisotropic images, changes in the scattering objects' orientation distribution probability (ODP) function are investigated as the temperature is lowered, thus triggering swelling of the individual particles by water intercalation. This is done, on the one hand, by inferring the width of the ODP function from the eccentricity of quasi-elliptic iso-intensity cuts of the small-angle scattering images, and, on the other hand, by obtaining the ODP function from azimuthal profiles of the images. The decays of the scattering intensity as a function of momentum transfer along the two principal directions of the images exhibit power law behaviors. A crossover scale between two power law regimes is observed on the profiles recorded along the horizontal axis; it corresponds to the typical pore size along the direction of the initially applied load. These results are compared with a previous study of similar systems
Student perceptions of ethics and professionalism in computer science: Does age, gender, or experience matter?
This work explores several factors that impact ethics education in undergraduate computer science, including: the overall understanding of ethics material, any differences between male and female subjects, any differences between traditional age and non-traditional age subjects, and any differences in experience level. Instruments (attitudinal surveys and reflective scenarios) were distributed to four institutions in the Midwest; 74 out of 170 were returned.
Female subjects demonstrated a markedly better ability to distinguish between ethical and unethical behavior in the scenarios (especially in areas where the distinctions were less obvious) and produced higher quality written justifications. There were only 4 attitudinal survey items where differences were found between the genders, each attributable to the different sizes of the groups (though the split, 63% male and 37% female, is fairly consistent with current computer science enrollment data).
Based on attitudinal survey responses, nontraditional students demonstrated a clearer understanding of the relationship between academic honesty and ethics than did traditional age students. Other differences existed, but were primarily due to the vastly different group sizes (84% traditional age, 16% nontraditional age). In the qualitative data, the two groups responded similarly to all but one scenario; the writing of the nontraditional students tended to be of more consistent quality.
Among the experience levels, differences were found on several survey items and in the response patterns on all of the scenario data. The same general trend evidenced itself in both cases; freshmen (those who have taken/completed less than 3 courses) displayed a greater ability to correctly distinguish ethical and unethical acts, and responded to the acts more clearly and concisely than their more experienced counterparts. The pattern declines steadily through the juniors (7 to 9 courses) and rebounds slightly for seniors (10 or more courses)
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