Methods for evaluating Decision Problems with Limited Information

LImited Memory Influence Diagrams (LIMIDs) are general models of decision problems for representing limited memory policies (Lauritzen and Nilsson (2001)). The evaluation of LIMIDs can be done by Single Policy Updating that produces a local maximum strategy in which no single policy modification can increase the expected utility. This paper examines the quality of the obtained local maximum strategy and proposes three different methods for evaluating LIMIDs. The first algorithm, Temporal Policy Updating, resembles Single Policy Updating. The second algorithm, Greedy Search, successively updates the policy that gives the highest expected utility improvement. The final algorithm, Simulating Annealing, differs from the two preceeding by allowing the search to take some downhill steps to escape a local maximum. A careful comparison of the algorithms is provided both in terms of the quality of the obtained strategies, and in terms of implementation of the algorithms including some considerations of the computational complexity

Butterfly Optics Exceed the Theoretical Limits of Conventional Apposition Eyes

Optical experiments on butterfly compound eyes show that they have angular sensitivities narrower than expected from conventional apposition eyes. This superior performance is explained by a theoretical model where the cone stalk is considered as a modecoupling device. In this model the Airy diffraction pattern of the corneal facet excites a combination of the two waveguide modes LP01 and LP02. When the two modes propagate through the cone stalk the power of LP02 is transferred to LP01 alone which is supported by the rhabdom. This mechanism produces a higher on-axis sensitivity and a narrower angular sensitivity than conventional apposition optics. Several predictions of the model were confirmed experimentally.

The micrometeoroid experiment on the OGO 2 satellite

Micrometeoroids in earths dust cloud obtained from OGO-B satellit

Intermittent Connectivity for Exploration in Communication-Constrained Multi-Agent Systems

Motivated by exploration of communication-constrained underground environments using robot teams, we study the problem of planning for intermittent connectivity in multi-agent systems. We propose a novel concept of information-consistency to handle situations where the plan is not initially known by all agents, and suggest an integer linear program for synthesizing information-consistent plans that also achieve auxiliary goals. Furthermore, inspired by network flow problems we propose a novel way to pose connectivity constraints that scales much better than previous methods. In the second part of the paper we apply these results in an exploration setting, and propose a clustering method that separates a large exploration problem into smaller problems that can be solved independently. We demonstrate how the resulting exploration algorithm is able to coordinate a team of ten agents to explore a large environment

Bottom-up derivation of an effective thermostat for united atoms simulations of water

In this article we derive the effective pairwise interactions in a Langevin type united atoms model of water. The interactions are determined from the trajectories of a detailed molecular dynamics simulation of simple point charge water. A standard method is used for estimating the conservative interaction, whereas a new "bottom-up" method is used to determine the effective dissipative and stochastic interactions. We demonstrate that, when compared to the standard united atoms model, the transport properties of the coarse-grained model is significantly improved by the introduction of the derived dissipative and stochastic interactions. The results are compared to a previous study, where a "top-down" approach was used to obtain transport properties consistent with those of the simple point charge water model.Comment: Submitted to J. Chem. Phy

Overcoming migration during giant planet formation

In the core accretion model, gas giant formation is a race between growth and migration; for a core to become a jovian planet, it must accrete its envelope before it spirals into the host star. We use a multizone numerical model to extend our previous investigation of the "window of opportunity" for gas giant formation within a disk. When the collision cross-section enhancement due to core atmospheres is taken into account, we find that a broad range of protoplanetary disks posses such a window.Comment: 4 pages, 3 figs, accepted to ApJ

Price Pooling and the Gains from Hedging: Application to a Swedish Grain Cooperative

Optimal hedging strategies are analyzed for a cooperative operating a price pooling system in the presence of price and quantity risk. A three-period model, accounting for default risk and storage, is developed. Hedging allows the cooperative to increase the pool price offered to farmers by 2.8 - 4% for moderate risk parameters.Agribusiness, Marketing,

Properties of optically selected BL Lac candidates from the SDSS

\textbf{Context.} Deep optical surveys open the avenue for find large numbers of BL Lac objects that are hard to identify because they lack the unique properties classifying them as such. While radio or X-ray surveys typically reveal dozens of sources, recent compilations based on optical criteria alone have increased the number of BL Lac candidates considerably. However, these compilations are subject to biases and may contain a substantial number of contaminating sources. \textbf{Aims.} In this paper we extend our analysis of 182 optically selected BL Lac object candidates from the SDSS with respect to an earlier study. The main goal is to determine the number of bona fide BL Lac objects in this sample. \textbf{Methods.} We examine their variability characteristics, determine their broad-band radio-UV SEDs, and search for the presence of a host galaxy. In addition we present new optical spectra for 27 targets with improved S/N with respect to the SDSS spectra. \textbf{Results.} At least 59% of our targets have shown variability between SDSS DR2 and our observations by more than 0.1-0.27 mag de- pending on the telescope used. A host galaxy was detected in 36% of our targets. The host galaxy type and luminosities are consistent with earlier studies of BL Lac host galaxies. Simple fits to broad-band SEDS for 104 targets of our sample derived synchrotron peak frequencies between $13.5 \leq \mathrm{log}_{10}(\nu_{\mathrm{peak}}) \leq 16$ with a peak at $\mathrm{log}_{10} \sim 14.5$. Our new optical spectra do not reveal any new redshift for any of our objects. Thus the sample contains a large number of bona fide BL Lac objects and seems to contain a substantial fraction of intermediate-frequency peaked BL Lacs.Comment: Accepted for publication in A\&

Comment on ``Two Time Scales and Violation of the Fluctuation-Dissipation Theorem in a Finite Dimensional Model for Structural Glasses''

In cond-mat/0002074 Ricci-Tersenghi et al. find two linear regimes in the fluctuation-dissipation relation between density-density correlations and associated responses of the Frustrated Ising Lattice Gas. Here we show that this result does not seem to correspond to the equilibrium quantities of the model, by measuring the overlap distribution P(q) of the density and comparing the FDR expected on the ground of the P(q) with the one measured in the off-equilibrium experiments.Comment: RevTeX, 1 page, 2 eps figures, Comment on F. Ricci-Tersenghi et al., Phys. Rev. Lett. 84, 4473 (2000

Photoreceptor Evolution: Ancient Siblings Serve Different Tasks

AbstractPhotoreceptor cells of vertebrate eyes are fundamentally different from those of invertebrate eyes. New work on the brain of a ragworm now suggests that ancestral bilaterians possessed both types of photoreceptor cell