15,385 research outputs found
Submicron metal powders produced by ball milling with grinding aids
In ball milling metal powders to submicron size, various salts are more effective as grinding aids than conventional surfactants. Absolute ethyl alcohol is used as the grinding liquid
Alternatives for jet engine control
Alternatives to linear quadratic regulator theory in the linear case are examined along with nonlinear modelling and optimization approaches for global control. Context for the studies has been set by the DYNGEN digital simulator and by models generated for various phases of the F100 Multivariable Control Synthesis Program. With respect to the linear alternatives, the multivariable frequency domain is stressed. Progress is reported in both the direct algebraic approach to exact model matching, by means of stimulating work on the basic computational issues, and in the indirect generalized Nyquist approach. With respect to nonlinear modelling and optimization, the emphasis is twofold: the development of analytical nonlinear models of the jet engine and the use of these models in conjunction with techniques of mathematical programming in order to study global control over nonincremental portions of the flight envelope. The possibility of using tensor methods is explored
A technique to correct for sample thickness variations for use with IDAPS X-ray radiograph analysis
The Image Data Processing System (IDAPS) at the Marshall Space Flight Center is used to analyze radiographs of metal samples to qualitatively and quantitatively map compositional variations across the samples. When the X-ray radiographs are of samples having thickness variations, corrections must be made to accomplish compositional analysis. A correction technique is described for cylindrical samples and is applied to radiographs of SPAR Experiment 74-18. Uncorrected and corrected images are shown
Asymmetry distributions and mass effects in dijet events at a polarized HERA
The asymmetry distributions for several kinematic variables are considered
for finding a systematic way to maximize the signal for the extraction of the
polarized gluon density. The relevance of mass effects for the corresponding
dijet cross section is discussed and the different approximations for including
mass effects are compared. We also compare via the programs Pepsi and Mepjet
two different Monte Carlo (MC) approaches for simulating the expected signal in
the dijet asymmetry at a polarized HERA.Comment: 18 pages, LaTeX, minor stylistic changes for Z.Phys.
High-Dimensional Density Ratio Estimation with Extensions to Approximate Likelihood Computation
The ratio between two probability density functions is an important component
of various tasks, including selection bias correction, novelty detection and
classification. Recently, several estimators of this ratio have been proposed.
Most of these methods fail if the sample space is high-dimensional, and hence
require a dimension reduction step, the result of which can be a significant
loss of information. Here we propose a simple-to-implement, fully nonparametric
density ratio estimator that expands the ratio in terms of the eigenfunctions
of a kernel-based operator; these functions reflect the underlying geometry of
the data (e.g., submanifold structure), often leading to better estimates
without an explicit dimension reduction step. We show how our general framework
can be extended to address another important problem, the estimation of a
likelihood function in situations where that function cannot be
well-approximated by an analytical form. One is often faced with this situation
when performing statistical inference with data from the sciences, due the
complexity of the data and of the processes that generated those data. We
emphasize applications where using existing likelihood-free methods of
inference would be challenging due to the high dimensionality of the sample
space, but where our spectral series method yields a reasonable estimate of the
likelihood function. We provide theoretical guarantees and illustrate the
effectiveness of our proposed method with numerical experiments.Comment: With supplementary materia
Map-Based Navigation in a Graphical MOO
Traditional MUDs and MOOs lack support for global wareness and simple navigation. These problems can be addressed by the introduction of a map-based navigation tool. In this paper we report on the design and evaluation of such a tool for MOOsburg, a graphical 2D MOO based on the town of Blacksburg, Virginia. The tool supports exploration and place-based tasks in the MOO. It also allows navigation of a large-scale map and encourages users to develop survey knowledge of the town. An evaluation revealed some initial usability problems with our prototype and suggested new design ideas that may better support users. Using these results, the lessons learned about map-based navigation are presented
On the structure of framed vertex operator algebras and their pointwise frame stabilizers
In this paper, we study the structure of a general framed vertex operator
algebra. We show that the structure codes (C,D) of a framed VOA V satisfy
certain duality conditions. As a consequence, we prove that every framed VOA is
a simple current extension of the associated binary code VOA V_C. This result
would give a prospect on the classification of framed vertex operator algebras.
In addition, the pointwise frame stabilizer of V is studied. We completely
determine all automorphisms in this pointwise stabilizer, which are of order 1,
2 or 4. The 4A-twisted sector and the 4A-twisted orbifold theory of the famous
Moonshine VOA are also constructed explicitly. We verify that the top module of
this twisted sector is of dimension 1 and of weight 3/4 and the VOA obtained by
4A-twisted orbifold construction of the moonshine VOA is isomorphic to the
moonshine VOA itself.Comment: Version 3: 59 pages. Corrected version. 54 pages on my LaTeX system
version 2: We add Theorem 5.16 in which we give a necessary and sufficient
condtion for a code to be a structure code of a holomorphic framed VOA.
"hyperref" style is also introduce
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