Position-specific induction of ectopic limbs in non-regenerating blastemas on axolotl forelimbs.

Ectopic retinoic acid (RA) has been hypothesized to reprogram the positional identity of cells in developing and regenerating limbs to a single positional value corresponding to the posterior-ventral-proximal (PVPr) position on the limb. We tested this hypothesis by using RA to reprogram the information of blastema cells that were induced to form at different positions around the limb circumference. We observed that RA treatment of blastemas in anterior and dorsal locations, but not posterior and ventral locations, resulted in the induction of complete ectopic limbs. These position-specific differences in limb induction are probably due to differences in the positional disparity between the RA-reprogrammed blastema cells and the cells at the periphery of the wound. These observations are consistent with the hypothesis that RA treatment reprograms the information in blastema cells to the PVPr position on the limb, since anterior and dorsal positions have the largest disparity and posterior and ventral have the smallest disparity from the PVPr identity

A Graph Model for Imperative Computation

Scott's graph model is a lambda-algebra based on the observation that continuous endofunctions on the lattice of sets of natural numbers can be represented via their graphs. A graph is a relation mapping finite sets of input values to output values. We consider a similar model based on relations whose input values are finite sequences rather than sets. This alteration means that we are taking into account the order in which observations are made. This new notion of graph gives rise to a model of affine lambda-calculus that admits an interpretation of imperative constructs including variable assignment, dereferencing and allocation. Extending this untyped model, we construct a category that provides a model of typed higher-order imperative computation with an affine type system. An appropriate language of this kind is Reynolds's Syntactic Control of Interference. Our model turns out to be fully abstract for this language. At a concrete level, it is the same as Reddy's object spaces model, which was the first "state-free" model of a higher-order imperative programming language and an important precursor of games models. The graph model can therefore be seen as a universal domain for Reddy's model

Low-energy finite field arithmetic primitives for implementing security in wireless sensor networks

In this paper we propose the use of identity based encryption (IBE) for ensuring a secure wireless sensor network. In this context we have implemented the arithmetic operations required for the most computationally expensive part of IBE, which is the Tate pairing, in 90 nm CMOS and obtained area, timing and energy figures for the designs. Initial results indicate that a hardware implementation of IBE would meet the strict energy constraint of a wireless sensor network nod

Active particle dynamics beyond the jamming density

Many biological systems form colonies at high density. Passive granular systems will be jammed at such densities, yet for the survival of biological systems it is crucial that they are dynamic. We construct a phase diagram for a system of active particles interacting via Vicsek alignment, and vary the density, self-propulsion force, and orientational noise. We find that the system exhibits four different phases, characterized by transitions in the effective diffusion constant and in the orientational order parameter. Our simulations show that there exists an optimal noise such that particles require a minimal force to unjam, allowing for rearrangements.Comment: 7 pages, 8 figure

Introduction: Peasants, Pastoralists and Proletarians: Joining the Debates on Trajectories of Agrarian Change, Livelihoods and Land Use

Recent changes in the agrarian studies and geography literatures present differing views on the pace and trajectory of change in rural developing areas. In this special section of Human Geography, we contrast the theoretical and practice implications of these differing approaches, namely depeasantization, accumulation by dispossession and deproletarianization. Depeasantization refers to change in livelihood activities out of agriculture, long theorized as necessary for an area’s transition into capitalism. Accumulation by dispossession is a process of on-going capital accumulation where a give resource is privatized, seized, or in some other manner alienated from common ownership in order to provide a basis for continued capital accumulation. Deproletarianization occurs when workers are no longer able to freely commodify and recommodify their only commodity, their own labour. In this section, we explore these three theses with case studies that draw upon empirical data. The papers in this collection all speak to one aspect or another of these debates. We do not intend to try to determine a “best approach”, rather we explore strengths and weaknesses of each argument. The production of nature, change in the mode of production and the political economy of nature are discussed in the first article by Brent McCusker. Phil O’Keefe and Geoff O’Brien examine the evolution of worked landscape under pre-capitalist modes of production in riverine ecologies. Through further case studies, Paul O’Keefe explores links between livelihoods and climate change in Mt. Kilimanjaro, Tanzania, while Franklin Graham explores the persistence of pastoralism in the Sahel. Finally, Naomi Shanguhyia and Brent McCusker examine the process of governance in dry land Kenya through the study of chronic food shortages

Efficient storage and decoding of SURF feature points

Practical use of SURF feature points in large-scale indexing and retrieval engines requires an efficient means for storing and decoding these features. This paper investigates several methods for compression and storage of SURF feature points, considering both storage consumption and disk-read efficiency. We compare each scheme with a baseline plain-text encoding scheme as used by many existing SURF implementations. Our final proposed scheme significantly reduces both the time required to load and decode feature points, and the space required to store them on disk