Geomorphology of Mountainous Deserts

The peculiar land forms of the desert are due largely (1) to the paucity of plant growth, in consequence of which (2) disintegrated rock, produced more by physical than by chemical processes, which (3) vary with rock nature (hence granitic rocks assume forms of special interest), does not remain near its source long enough in the earlier stages of erosion to be reduced to very fine texture before (4) it is carried by stream floods or sheet floods toward or to (5) playa basins of rising base-level; but in an advanced stage of the arid cycle (6) the playa surface may be lowered by the wind, whereupon the rock slopes previously weathered down to sheet-flood grade and buried with sheet-flood waste will be (7) trenched by stream floods with respect to the sinking baselevel, and the buried rock floors will be laid bare until again reduced to sheet-flood grade. But (8) in deserts draining to the ocean these various processes will work with respect to a relatively fixed baselevel; also (9) in deserts where the action of water floods is dominated by wind action, the carving of the surface will be in part independent of any baselevel

On Normal Subgroups of Coxeter Groups Generated by Standard Parabolic Subgroups

We discuss one construction of nonstandard subgroups in the category of Coxeter groups. Two formulae for the growth series of such a subgroups are given. As an application we construct a flag simple convex polytope, whose f-polynomial has non-real roots.Comment: 12 pages, figure

Vanishing results for the cohomology of complex toric hyperplane complements

Suppose \Cal R is the complement of an essential arrangement of toric hyperlanes in the complex torus (\C^*)^n and \pi=\pi_1(\Cal R). We show that H^*(\Cal R;A) vanishes except in the top degree $n$ when $A$ is one of the following systems of local coefficients: (a) a system of nonresonant coefficients in a complex line bundle, (b) the von Neumann algebra \cn\pi, or (c) the group ring \zz \pi. In case (a) the dimension of $H^n$ is |e(\Cal R)| where e(\Cal R) denotes the Euler characteristic, and in case (b) the $n^{\mathrm{th}}$ \eltwo Betti number is also |e(\Cal R)|.Comment: 14 pages. arXiv admin note: substantial text overlap with arXiv:math/061240

Existence versus Exploitation: The Opacity of Backbones and Backdoors Under a Weak Assumption

Backdoors and backbones of Boolean formulas are hidden structural properties. A natural goal, already in part realized, is that solver algorithms seek to obtain substantially better performance by exploiting these structures. However, the present paper is not intended to improve the performance of SAT solvers, but rather is a cautionary paper. In particular, the theme of this paper is that there is a potential chasm between the existence of such structures in the Boolean formula and being able to effectively exploit them. This does not mean that these structures are not useful to solvers. It does mean that one must be very careful not to assume that it is computationally easy to go from the existence of a structure to being able to get one's hands on it and/or being able to exploit the structure. For example, in this paper we show that, under the assumption that P $\neq$ NP, there are easily recognizable families of Boolean formulas with strong backdoors that are easy to find, yet for which it is hard (in fact, NP-complete) to determine whether the formulas are satisfiable. We also show that, also under the assumption P $\neq$ NP, there are easily recognizable sets of Boolean formulas for which it is hard (in fact, NP-complete) to determine whether they have a large backbone

The Numerical Studies Program for the Atmospheric General Circulation Experiment (AGCE) for Spacelab Flights

The atmospheric general circulation experiment (AGCE) numerical design for Spacelab flights was studied. A spherical baroclinic flow experiment which models the large scale circulations of the Earth's atmosphere was proposed. Gravity is simulated by a radial dielectric body force. The major objective of the AGCE is to study nonlinear baroclinic wave flows in spherical geometry. Numerical models must be developed which accurately predict the basic axisymmetric states and the stability of nonlinear baroclinic wave flows. A three dimensional, fully nonlinear, numerical model and the AGCE based on the complete set of equations is required. Progress in the AGCE numerical design studies program is reported

Analysis of potential helicopter vibration reduction concepts

Results of analytical investigations to develop, understand, and evaluate potential helicopter vibration reduction concepts are presented in the following areas: identification of the fundamental sources of vibratory loads, blade design for low vibration, application of design optimization techniques, active higher harmonic control, blade appended aeromechanical devices, and the prediction of vibratory airloads. Primary sources of vibration are identified for a selected four-bladed articulated rotor operating in high speed level flight. The application of analytical design procedures and optimization techniques are shown to have the potential for establishing reduced vibration blade designs through variations in blade mass and stiffness distributions, and chordwise center-of-gravity location

Generalized linear stability of noninertial coating flows over topographical features

The transient evolution of perturbations to steady lubrication flow over a topographically patterned surface is investigated via a nonmodal linear stability analysis of the non-normal disturbance operator. In contrast to the capillary ridges that form near moving contact lines, the stationary capillary ridges near trenches or elevations have only stable eigenvalues. Minimal transient amplification of perturbations occurs, regardless of the magnitude or steepness of the topographical features. The absence of transient amplification and the stability of the ridge are explained on physical grounds. By comparison to unstable ridge formation on smooth, flat, and homogeneous surfaces, the lack of closed, recirculating streamlines beneath the capillary ridge is linked to the linear stability

Calibration of Hewlett-Packard network analyzers: A precision viewpoint

Alternative calibration procedures are examined for Hewlett-Packard vector network analyzers which lead to an improved open-circuit capacitance model, and hence, higher measurement accuracy

Molecular mechanisms that stabilize short term synaptic plasticity during presynaptic homeostatic plasticity.

Presynaptic homeostatic plasticity (PHP) compensates for impaired postsynaptic neurotransmitter receptor function through a rapid, persistent adjustment of neurotransmitter release, an effect that can exceed 200%. An unexplained property of PHP is the preservation of short-term plasticity (STP), thereby stabilizing activity-dependent synaptic information transfer. We demonstrate that the dramatic potentiation of presynaptic release during PHP is achieved while simultaneously maintaining a constant ratio of primed to super-primed synaptic vesicles, thereby preserving STP. Mechanistically, genetic, biochemical and electrophysiological evidence argue that a constant ratio of primed to super-primed synaptic vesicles is achieved by the concerted action of three proteins: Unc18, Syntaxin1A and RIM. Our data support a model based on the regulated availability of Unc18 at the presynaptic active zone, a process that is restrained by Syntaxin1A and facilitated by RIM. As such, regulated vesicle priming/super-priming enables PHP to stabilize both synaptic gain and the activity-dependent transfer of information at a synapse

The development of a pseudo-nyquist analysis technique for hybrid sampled-data control systems

The stability characteristics of a launch vehicle, as a function of gain and phase variations at the thrust vector controller, cannot be obtained using classical sampled-data control theory if the launch vehicle attitude control system contains both sampled-data and continuous feedback control loops. A method was developed which can be used to generate a sampled-data pseudo-Nyquist plot for gain and phase variations at the controller. This method was developed and used to determine the stability characteristics of the Saturn 1B launch vehicle in the backup guidance mode