A difference in expressive power between flowcharts and recursion schemes

We show the existence of a single interpretation for which no flowchart produces the same results as a particular recursion scheme.Partially supported by NSF Grant MCS77-15628.Partially supported by NSF Grant DCR75-02373.     

The self-consistent equilibrium and diffusion code sced

The SCED code solves the equilibrium equations for a Tokamak axisymmetric configuration and the plasma diffusion equations. The equilibrium system takes into account the presence of a magnetic circuit made of iron, the currents in the poloidal coils and the plasma pressure and toroidal field profiles. The plasma boundary is a free boundary defined by its contact with a limiter. A variational formulation of the problem is written and the equations of the poloidal flux ψ are solved by a finite element method; the Picard and the Newton algorithms are tested for the treatment of the non-linearities and compared on TFR and JET equilibrium configurations. The variation of the plasma position with respect to the currents in the coils is then studied by a continuation method and the link between the convergence of the Picard method and the stability of the plasma motions is proved. The evolution problem takes into account the induction of the primary coils and the penetration of the eddy currents in the vacuum vessel. The 2D diffusion equations for the electronic and ionic densities and temperatures and for the poloidal flux are written in terms of the toroidal flux by averaging over the magnetic surfaces. These equations are solved by an implicit scheme in time and by a finite difference scheme in space; the non-linearities are treated by the Newton method. The coupling between the equilibrium system and the diffusion equations enables one to follow the evolution of the plasma boundary. The study of the plasma motions in TFR, in the presence or absence of a feedback system, has been performed with this code and compared with the experimental observations.     

A population-based iterated greedy algorithm for the minimum weight vertex cover problem

Given an undirected, vertex-weighted graph, the goal of the minimum weight vertex cover problem is to find a subset of the vertices of the graph such that the subset is a vertex cover and the sum of the weights of its vertices is minimal. This problem is known to be NP-hard and no efficient algorithm is known to solve it to optimality. Therefore, most existing techniques are based on heuristics for providing approximate solutions in a reasonable computation time.Population-based search approaches have shown to be effective for solving a multitude of combinatorial optimization problems. Their advantage can be identified as their ability to find areas of the space containing high quality solutions. This paper proposes a simple and efficient population-based iterated greedy algorithm for tackling the minimum weight vertex cover problem. At each iteration, a population of solutions is established and refined using a fast randomized iterated greedy heuristic based on successive phases of destruction and reconstruction. An extensive experimental evaluation on a commonly used set of benchmark instances shows that our algorithm outperforms current state-of-the-art approaches.     

Advances in selective wet oxidation of AlGaAs alloys

We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation. Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As₂O₃ to As. The stable oxide is amorphous (Al_xGa_1-x) ₂O₃ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage. The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process. Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found. The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content. Oxidation selectivity to thickness is also shown for layer thickness <60 nm. Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported     

Multisite microprobes for neural recordings

Multisite, passive microprobes have been developed to allow simultaneous recording of action potential activity from multiple neurons at different locations in the brain. The microprobes were fabricated using standard integrated circuit techniques. The probe is a planar structure that consists of gold electrodes sandwiched between two polyimide dielectric layers and bonded to a molybdenum structural support. Windows in the top dielectric layer expose the electrode sites and bonding pads. In two distinct versions of the probe four or six recordings sites, respectively, of approximately 25 microns 2 are arranged on a dagger-shaped structure which can penetrate the pia. The bonding pads and interconnect wires at the probe head are entirely encapsulated in a tubular fixture that is packed with silicone RTV and sealed with epoxy to protect the interconnections from contact with body fluids. The site impedances at 1 kHz are typically between 2 and 4 M omega. Probe lifetimes for continuous immersion in physiological saline solution, as measured by impedance, have exceeded 750 h. The failure mechanism is believed to be due to moisture and ion absorption in the top dielectric layer. In acute neurophysiological experiments using the four site probes, action potential activity was recorded from physiologically identified neurons in the dorsal column nuclei of anesthetized rat.     

Delay-limited throughput of ad hoc networks

The delay-limited throughput of an ad hoc wireless network confined to a finite region is investigated. An approximate expression for the achievable throughput as a function of the maximum allowable delay is obtained. It is found that: 1) for moderate values of the delay d, the throughput that can be achieved by taking advantage of the motion increases as d/sup 2/3/ and 2) for a fixed value of d, the dependence of the achievable throughput on the number of nodes n is n/sup -1/3/. A transmission and relaying strategy ensuring continuous information flow is constructed. It is shown that there exists a critical value of the delay such that: 1) for values of the delay d below the critical delay, the throughput does not benefit appreciably from the motion and 2) the dependence of the critical delay on the number of nodes is a very slowly increasing function (n/sup 1/14/). Finally, asymptotic optimality of the proposed strategy in a certain class is shown.