Stiffness pathologies in discrete granular systems: bifurcation, neutral equilibrium, and instability in the presence of kinematic constraints

The paper develops the stiffness relationship between the movements and forces among a system of discrete interacting grains. The approach is similar to that used in structural analysis, but the stiffness matrix of granular material is inherently non-symmetric because of the geometrics of particle interactions and of the frictional behavior of the contacts. Internal geometric constraints are imposed by the particles' shapes, in particular, by the surface curvatures of the particles at their points of contact. Moreover, the stiffness relationship is incrementally non-linear, and even small assemblies require the analysis of multiple stiffness branches, with each branch region being a pointed convex cone in displacement-space. These aspects of the particle-level stiffness relationship gives rise to three types of micro-scale failure: neutral equilibrium, bifurcation and path instability, and instability of equilibrium. These three pathologies are defined in the context of four types of displacement constraints, which can be readily analyzed with certain generalized inverses. That is, instability and non-uniqueness are investigated in the presence of kinematic constraints. Bifurcation paths can be either stable or unstable, as determined with the Hill-Bazant-Petryk criterion. Examples of simple granular systems of three, sixteen, and sixty four disks are analyzed. With each system, multiple contacts were assumed to be at the friction limit. Even with these small systems, micro-scale failure is expressed in many different forms, with some systems having hundreds of micro-scale failure modes. The examples suggest that micro-scale failure is pervasive within granular materials, with particle arrangements being in a nearly continual state of instability

Transverse Instability of Avalanches in Granular Flows down Incline

Avalanche experiments on an erodible substrate are treated in the framework of ``partial fluidization'' model of dense granular flows. The model identifies a family of propagating soliton-like avalanches with shape and velocity controlled by the inclination angle and the depth of substrate. At high inclination angles the solitons display a transverse instability, followed by coarsening and fingering similar to recent experimental observation. A primary cause for the transverse instability is directly related to the dependence of soliton velocity on the granular mass trapped in the avalanche.Comment: 3 figures, 4 pages, submitted to Phys Rev Let

Gendered Impact of Irrigated Rice Schemes’ Governance on Farmers’ Income, Productivity and Technical Efficiency in Benin

Collective actions groups have many advantages and are sometimes essential, yet they can reinforce or perpetuate inter-and intra-gender inequalities when their functioning is left entirely subject to internal community dynamics and they are not well managed. This is well illustrated by the case of Koussin-Lélé rice scheme in the central Benin. This paper apply inequality indices and frontier production function to data from a sample of male and women rice farmers to analyze the gender inequalities in access to land and the governance of the groups, and their gender-differentiated impacts on farmers' productivity, technical efficiency and income. The results show that women are particularly discriminated against with regards to access to land, with significant negative impacts on their productivities and incomes. However, this discrimination did not have a significant impact on technical efficiency.Gender, land distribution, Rice, technical efficiency, productivity, income, Agricultural and Food Policy, Consumer/Household Economics, Demand and Price Analysis, Farm Management, Food Consumption/Nutrition/Food Safety, Food Security and Poverty, International Relations/Trade, Marketing, Productivity Analysis, Research and Development/Tech Change/Emerging Technologies,

Observations of the Structure and Dynamics of the Inner M87 Jet

M87 is the best source in which to study a jet at high resolution in gravitational units because it has a very high mass black hole and is nearby. The angular size of the black hole is second only to Sgr A*, which does not have a strong jet. The jet structure is edge brightened with a wide opening angle base and a weak counterjet. We have roughly annual observations for 17 years plus intensive monitoring at three week intervals for a year and five day intervals for 2.5 months made with the Very Long Baseline Array (VLBA) at 43 GHz. The inner jet shows very complex dynamics, with apparent motions both along and across the jet. Speeds from zero to over 2c are seen, with acceleration observed over the first 3 milli-arcseconds. The counterjet decreases in brightness much more rapidly than the main jet, as is expected from relativistic beaming in an accelerating jet oriented near the line-of-sight. Details of the structure and dynamics are discussed. The roughly annual observations show side-to-side motion of the whole jet with a characteristic time scale of about 9 years.Comment: 11 pages, 7 figures. Published in a special issue of Galaxies which is the proceedings of "Blazars through Sharp Multi-Wavelength Eyes" edited by J. L. Gomez, A. P. Marscher, and S. G. Jorsta

Monitoring edge-geodetic sets in graphs

We introduce a new graph-theoretic concept in the area of network monitoring. In this area, one wishes to monitor the vertices and/or the edges of a network (viewed as a graph) in order to detect and prevent failures. Inspired by two notions studied in the literature (edge-geodetic sets and distance-edge-monitoring sets), we define the notion of a monitoring edge-geodetic set (MEG-set for short) of a graph $G$ as an edge-geodetic set $S\subseteq V(G)$ of $G$ (that is, every edge of $G$ lies on some shortest path between two vertices of $S$) with the additional property that for every edge $e$ of $G$, there is a vertex pair $x, y$ of $S$ such that $e$ lies on \emph{all} shortest paths between $x$ and $y$. The motivation is that, if some edge $e$ is removed from the network (for example if it ceases to function), the monitoring probes $x$ and $y$ will detect the failure since the distance between them will increase. We explore the notion of MEG-sets by deriving the minimum size of a MEG-set for some basic graph classes (trees, cycles, unicyclic graphs, complete graphs, grids, hypercubes,...) and we prove an upper bound using the feedback edge set of the graph

From Complexity to Algebra and Back: Digraph Classes, Collapsibility, and the PGP

Inspired by computational complexity results for the quantified constraint satisfaction problem, we study the clones of idem potent polymorphisms of certain digraph classes. Our first results are two algebraic dichotomy, even "gap", theorems. Building on and extending [Martin CP'11], we prove that partially reflexive paths bequeath a set of idem potent polymorphisms whose associated clone algebra has: either the polynomially generated powers property (PGP), or the exponentially generated powers property (EGP). Similarly, we build on [DaMM ICALP'14] to prove that semi complete digraphs have the same property. These gap theorems are further motivated by new evidence that PGP could be the algebraic explanation that a QCSP is in NP even for unbounded alternation. Along the way we also effect a study of a concrete form of PGP known as collapsibility, tying together the algebraic and structural threads from [Chen Sicomp'08], and show that collapsibility is equivalent to its Pi2-restriction. We also give a decision procedure for k-collapsibility from a singleton source of a finite structure (a form of collapsibility which covers all known examples of PGP for finite structures). Finally, we present a new QCSP trichotomy result, for partially reflexive paths with constants. Without constants it is known these QCSPs are either in NL or Pspace-complete [Martin CP'11], but we prove that with constants they attain the three complexities NL, NP-complete and Pspace-complete

Statistical mechanics of lossy compression for non-monotonic multilayer perceptrons

A lossy data compression scheme for uniformly biased Boolean messages is investigated via statistical mechanics techniques. We utilize tree-like committee machine (committee tree) and tree-like parity machine (parity tree) whose transfer functions are non-monotonic. The scheme performance at the infinite code length limit is analyzed using the replica method. Both committee and parity treelike networks are shown to saturate the Shannon bound. The AT stability of the Replica Symmetric solution is analyzed, and the tuning of the non-monotonic transfer function is also discussed.Comment: 29 pages, 7 figure

E-Pad: Large Display Pointing in a Continuous Interaction Space around a Mobile Device

International audienceRelative pointing through using tactile mobile device (such as tablets of phones) on a large display is a viable interaction technique (that we call Pad in this paper) which permits accurate pointing. However, limited device size has consequences on interaction. Such systems are known to often require clutching, which degrades performances. We present E-Pad, an indirect relative pointing interaction technique which takes benefit of the mobile tactile surface combined with its surrounding space. A user can perform continuous relative pointing starting on the pad then continuing in the free space around the pad, within arm's reach. As a first step toward E-Pad, we first introduce extended continuous relative pointing gestures and conduct a preliminary study to determine how people move their hand around the mobile device. We then conduct an experiment that compares the performance of E-Pad and Pad. Our findings indicate that E-Pad is faster than Pad and decreases the number of clutches without compromising accuracy. Our findings also suggest an overwhelming preference for E-Pad

Improving Soil Health of Commercial Vegetable Home Gardens through Conservation Agriculture in Cambodia

Tillage systems are components of broad agricultural practices that affect soil properties and soil health. These changes include soil respiration, density, moisture, and pH. Conservation agriculture practices have the potential to improve soil health by reducing tillage. In agricultural production, there can be numerous approaches to achieving consistently high yields annually; however, this study specifically looked at conventional tillage and conservation agriculture systems. This study aimed to determine soil fauna biodiversity and soil health under conservation agriculture (CA) and conventional tillage (CT) management practices of vegetable production in Cambodia. Five CA and five CT plots were selected and included in this study. Fifty soil samples were collected from CA and CT plots for soil fauna measurement, and in-situ tests were made using Biofunctool© for soil health assessment. The results showed that the abundance of soil fauna and aggregation stability were greater in CA than in CT. Soil fauna biodiversity enhancement may provide better soil health for soil improvement by adapting farming management practices