The agrin gene codes for a family of basal lamina proteins that differ in function and distribution

We isolated two cDNAs that encode isoforms of agrin, the basal lamina protein that mediates the motor neuron-induced aggregation of acetylcholine receptors on muscle fibers at the neuromuscular junction. Both proteins are the result of alternative splicing of the product of the agrin gene, but, unlike agrin, they are inactive in standard acetylcholine receptor aggregation assays. They lack one (agrin-related protein 1) or two (agrin-related protein 2) regions in agrin that are required for its activity. Expression studies provide evidence that both proteins are present in the nervous system and muscle and that, in muscle, myofibers and Schwann cells synthesize the agrin-related proteins while the axon terminals of motor neurons are the sole source of agrin

The Computational Power of Optimization in Online Learning

We consider the fundamental problem of prediction with expert advice where the experts are "optimizable": there is a black-box optimization oracle that can be used to compute, in constant time, the leading expert in retrospect at any point in time. In this setting, we give a novel online algorithm that attains vanishing regret with respect to $N$ experts in total $\widetilde{O}(\sqrt{N})$ computation time. We also give a lower bound showing that this running time cannot be improved (up to log factors) in the oracle model, thereby exhibiting a quadratic speedup as compared to the standard, oracle-free setting where the required time for vanishing regret is $\widetilde{\Theta}(N)$. These results demonstrate an exponential gap between the power of optimization in online learning and its power in statistical learning: in the latter, an optimization oracle---i.e., an efficient empirical risk minimizer---allows to learn a finite hypothesis class of size $N$ in time $O(\log{N})$. We also study the implications of our results to learning in repeated zero-sum games, in a setting where the players have access to oracles that compute, in constant time, their best-response to any mixed strategy of their opponent. We show that the runtime required for approximating the minimax value of the game in this setting is $\widetilde{\Theta}(\sqrt{N})$, yielding again a quadratic improvement upon the oracle-free setting, where $\widetilde{\Theta}(N)$ is known to be tight

Agrin isoforms and their role in synaptogenesis

Agrin is thought to mediate the motor neuron-induced aggregation of synaptic proteins on the surface of muscle fibers at neuromuscular junctions. Recent experiments provide direct evidence in support of this hypothesis, reveal the nature of agrin immunoreactivity at sites other than neuromuscular junctions, and have resulted in findings that are consistent with the possibility that agrin plays a role in synaptogenesis throughout the nervous system

An efficient algorithm for learning with semi-bandit feedback

We consider the problem of online combinatorial optimization under semi-bandit feedback. The goal of the learner is to sequentially select its actions from a combinatorial decision set so as to minimize its cumulative loss. We propose a learning algorithm for this problem based on combining the Follow-the-Perturbed-Leader (FPL) prediction method with a novel loss estimation procedure called Geometric Resampling (GR). Contrary to previous solutions, the resulting algorithm can be efficiently implemented for any decision set where efficient offline combinatorial optimization is possible at all. Assuming that the elements of the decision set can be described with d-dimensional binary vectors with at most m non-zero entries, we show that the expected regret of our algorithm after T rounds is O(m sqrt(dT log d)). As a side result, we also improve the best known regret bounds for FPL in the full information setting to O(m^(3/2) sqrt(T log d)), gaining a factor of sqrt(d/m) over previous bounds for this algorithm.Comment: submitted to ALT 201

On the constitution of sodium at higher densities

Using density functional theory the atomic and electronic structure of sodium are predicted to depart substantially from those expected of simple metals for $r_s 130$ GPa). Newly-predicted phases include those with low structural symmetry, semi-metallic electronic properties (including zero-gap semiconducting limiting behavior), unconventional valence charge density distributions, and even those that raise the possibility of superconductivity, all at currently achievable pressures. Important differences emerge between sodium and lithium at high densities, and these are attributable to corresponding differences in their respective cores.Comment: 13 pages; 3 figure

Similarities between the Hubbard and Periodic Anderson Models at Finite Temperatures

The single band Hubbard and the two band Periodic Anderson Hamiltonians have traditionally been applied to rather different physical problems - the Mott transition and itinerant magnetism, and Kondo singlet formation and scattering off localized magnetic states, respectively. In this paper, we compare the magnetic and charge correlations, and spectral functions, of the two systems. We show quantitatively that they exhibit remarkably similar behavior, including a nearly identical topology of the finite temperature phase diagrams at half-filling. We address potential implications of this for theories of the rare earth ``volume collapse'' transition.Comment: 4 pages (RevTeX) including 4 figures in 7 eps files; as to appear in Phys. Rev. Let

Low-frequency incommensurate magnetic response in strongly correlated systems

It is shown that in the t-J model of Cu-O planes at low frequencies the dynamic spin structure factor is peaked at incommensurate wave vectors (1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with the momentum dependencies of the magnon frequency and damping near the antiferromagnetic wave vector. The behavior of the incommensurate peaks is similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}: for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional to x, while for x>0.12 it tends to saturation. The incommensurability disappears with increasing temperature. Generally the incommensurate magnetic response is not accompanied by an inhomogeneity of the carrier density.Comment: 4 pages, 4 figure

Survey and Ecological Studies of the Termites (Isoptera: Kalotermitidae) of Mona Island

A survey of eight sites on Mona Island revealed four termite species in the family Kalotermitidae: Incisitermes nr. bequaerti (Snyder), I. nr. incisus (Silvestri), Neotermes mona (Banks), and Procryptotermes corniceps (Snyder). Incisitermes nr. bequaerti is a new record for the island. Identifiable wood hosts are reported. All species were found in dead wood, which is typical of drywood termites, but N. mona and P. corniceps were also collected from live wood. Examination of dead wood in three 9.3 m2 plots in a dense stand of Leucaena leucocephala (Lam.) de Wit. revealed that an average of approximately three-quarters of the wood pieces were attacked by termites. Although the total volume of dead wood was almost six times greater in Plot 1 than in the other two plots, the number of termites per unit volume of dead wood was very similar (0.4 per cm2). Total numbers of individuals per colony ranged from 11 to 3,359 termites. Caste composition is reported for each colony, and large variations among colonies were noted. Alates as well as eggs were more common in larger colonies. Soldier percentages ranged from 0.7% to 20.5% in 16 P. corniceps colonies. The largest colonies occurred when the greatest volume of dead wood was available. /// Un muestreo de ocho sitios en la Isla Mona reveló cuatro especies de termitas de la familia Kalotermitidae: Incisitermes nr. bequaerti (Snyder), I. nr. incisus (Silvestri), Neotermes mona (Banks), y Procryptotermes corniceps (Snyder). Incisitermes nr. bequaerti es un nuevo registro para la isla. Son reportadas las maderas hospedantes. Todas las especies fueron encontradas en madera muerta, lo cual es típico para las termitas de madera seca, pero N. mona y P. corniceps fueron también colectados en madera viva. El exámen de la madera muerta en tres parcelas de 9.3m2 en un denso grupo de Leucaena leucocephala (Lam.) de Wit. reveló que un promedio de aproximadamente tres cuartos de los pedazos de madera estaban atacados por las termitas. A pesar de que el volumen total de madera muerta fué casi seis veces mayor en la parcela 1 que en las otras dos, el número total de termitas por unidad de volumen de madera muerta fue muy similar (0.4/cm3). El número total de individuos por colonia varió de 11 a 3359 termitas. La composición de las castas es reportada para cada colonia. Fueron observadas grandes variaciones dentro de las colonias. Los porcentajes de soldados variaron de 0.7 a 20.5% en 16 colonias de P. corniceps. Las mayores colonias fueron encontradas cuando el mayor volumen de madera muerta era disponible

Phase separation and valence instabilities in cuprate superconductors. Effective one-band model approach

We study the Cu-O valence instability (VI) and the related phase separation (PS) driven by Cu-O nearest-neighbor repulsion $U_{pd}$, using an effective extended one-band Hubbard model ($H_{eff}$) obtained from the extended three-bandHubbard model, through an appropriate low-energy reduction. $H_{eff}$ is solved by exact diagonalization of a square cluster with 10 unit cells and also within a slave-boson mean-field theory. Its parameters depend on doping for $U_{pd}\neq 0$ or on-site O repulsion $U_p\neq 0$. The results using both techniques coincide in that there is neither VI nor PS for doping levels $x<0.5$ if $U_{pd}\lesssim 2$ eV. The PS region begins for $U_{pd}\gtrsim 2$ eV at large doping $x>0.6$ and increases with increasing $U_{pd}$. The PS also increases with increasing on-site Cu repulsion $U_d$.Comment: 16 pages and 10 figures in postscript format, compressed with uufile