Reachability in Higher-Order-Counters

Higher-order counter automata (\HOCS) can be either seen as a restriction of higher-order pushdown automata (\HOPS) to a unary stack alphabet, or as an extension of counter automata to higher levels. We distinguish two principal kinds of \HOCS: those that can test whether the topmost counter value is zero and those which cannot. We show that control-state reachability for level $k$ \HOCS with $0$-test is complete for \mbox{$(k-2)$}-fold exponential space; leaving out the $0$-test leads to completeness for \mbox{$(k-2)$}-fold exponential time. Restricting \HOCS (without $0$-test) to level $2$, we prove that global (forward or backward) reachability analysis is \PTIME-complete. This enhances the known result for pushdown systems which are subsumed by level $2$ \HOCS without $0$-test. We transfer our results to the formal language setting. Assuming that \PTIME \subsetneq \PSPACE \subsetneq \mathbf{EXPTIME}, we apply proof ideas of Engelfriet and conclude that the hierarchies of languages of \HOPS and of \HOCS form strictly interleaving hierarchies. Interestingly, Engelfriet's constructions also allow to conclude immediately that the hierarchy of collapsible pushdown languages is strict level-by-level due to the existing complexity results for reachability on collapsible pushdown graphs. This answers an open question independently asked by Parys and by Kobayashi.Comment: Version with Full Proofs of a paper that appears at MFCS 201

Wear and Friction Behavior of Pressure Infiltration Cast Copper-Carbon Composites

Metal matrix composites, prepared by pressure infiltration casting of copper base alloy into 2 types of microporous carbon preforms, one with 100% amorphous carbon and the other containing 40 wt pct. graphite and 60 wt pct. amorphous carbon, have been examined for their wear and friction behavior under ambient conditions using a pin-on-plate reciprocating wear tester. Micro-structural characterization of tribo-surfaces has been carried out. The thin carbon films formed on the tribo-surface reduced the friction coefficient and wear for the composites, especially at low loads on the pin. Adhesive wear observed on the tribo-surface promoted wear and friction at high loads. The wear and friction were observed to be very sensitive to the size and distribution of the microstructural constituents

México: entre el surrealismo de lo chafa y la farsa y la utopía

Es innegable que la violencia, en sus múltiples dimensiones y manifestaciones, sigue prevaleciendo en los países de la región latinoamericana. Y la violencia sólo genera más violencia. Siendo ello así, por diversas y muy particulares condiciones, México ha sido siempre un referente de y para la región. Sin embargo, esas mismas peculiaridades, han hecho que México pierda su papel protagónico. De tal manera que hoy, México ha sido desplazado de aquel liderazgo referencial que el país representaba para la región latinoamericana y para el mundo entero, debido a sus incongruencias, excesos y decisiones erráticas. En una palabra, por sus chafeadas y sus farsas. Y no es para menos, toda vez que, entre otros países, Bolivia, Brasil, Uruguay e incluso la Venezuela de Maduro, por diversas razones emblemáticas, son ahora el foco de atención y representatividad de la zona, gracias a la reivindicación de la dignidad y la identidad de sus respectivas naciones. De aquí, entonces, la pretensión de estas líneas por realizar una semblanza del actual acontecer mexicano, donde la utopía de la democracia continúa en entredicho y el país sigue debatiéndose entre el surrealismo que siempre lo ha caracterizado, y que hoy se encuentra nuevamente bajo los auspicios de un régimen protagónico, autoritario y fanfarrón

Consistent Treatment of Relativistic Effects in Electrodisintegration of the Deuteron

The influence of relativistic contributions to deuteron electrodisintegration is systematically studied in various kinematic regions of energy and momentum transfer. As theoretical framework the equation-of-motion and the unitarily equivalent S-matrix approaches are used. In a (p/M)-expansion, all leading order relativistic $\pi$-exchange contributions consistent with the Bonn OBEPQ model are included. In addition, static heavy meson exchange currents including boost terms, $\gamma\pi\rho/\omega$-currents, and $\Delta$-isobar contributions are considered. Sizeable effects from the various relativistic two-body contributions, mainly from $\pi$-exchange, have been found in inclusive form factors and exclusive structure functions for a variety of kinematic regions.Comment: 41 pages revtex including 15 postscript figure

Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?

The STEREO/WAVES instrument has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.Comment: In press in Solar Physics, 13 pages, 5 figure