247 research outputs found
Structural and ultrametric properties of twenty(L-alanine)
We study local energy minima of twenty(L-alanine). The minima are generated
using high-temperature Molecular Dynamics and Chain-Growth Monte Carlo
simulations, with subsequent minimization. We find that the lower-energy
configurations are -helices for a wide range of dielectric constant
values and that there is no noticeable difference
between the distribution of energy minima in space for different
values of Ultrametricity tests show that lower-energy -helical) configurations form a set which is ultrametric to a
certain degree, providing evidence for the presence of fine structure among
those minima. We put forward a heuristic argument for this fine structure. We
also find evidence for ultrametricity of a different kind among and energy minima. We analyze the distribution of lengths of
-helical portions among the minimized configurations and find a
persistence phenomenon for the ones, in qualitative agreement
with previous studies of critical lengths. Email contact:
[email protected]: Saclay-T93/025 Email: [email protected]
Computer-assisted access to the kidney
OBJECTIVES: The aim of this paper is to introduce the principles of
computer-assisted access to the kidney. The system provides the surgeon with a
pre-operative 3D planning on computed tomography (CT) images. After a rigid
registration with space-localized ultrasound (US) data, preoperative planning
can be transferred to the intra-operative conditions and an intuitive
man-machine interface allows the user to perform a puncture. MATERIAL AND
METHODS: Both CT and US images of informed normal volunteer were obtained to
perform calculation on the accuracy of registration and punctures were carried
out on a kidney phantom to measure the precision of the whole of the system.
RESULTS: We carried out millimetric registrations on real data and guidance
experiments on a kidney phantom showed encouraging results of 4.7 mm between
planned and reached targets. We noticed that the most significant error was
related to the needle deflection during the puncture. CONCLUSION: Preliminary
results are encouraging. Further work will be undertaken to improve efficiency
and accuracy, and to take breathing into account
Field theoretic approach to the counting problem of Hamiltonian cycles of graphs
A Hamiltonian cycle of a graph is a closed path that visits each site once
and only once. I study a field theoretic representation for the number of
Hamiltonian cycles for arbitrary graphs. By integrating out quadratic
fluctuations around the saddle point, one obtains an estimate for the number
which reflects characteristics of graphs well. The accuracy of the estimate is
verified by applying it to 2d square lattices with various boundary conditions.
This is the first example of extracting meaningful information from the
quadratic approximation to the field theory representation.Comment: 5 pages, 3 figures, uses epsf.sty. Estimates for the site entropy and
the gamma exponent indicated explicitl
La Navarre française
Digitalización. Vitoria-Gasteiz : Archivos y Bibliotecas, Abril 1994Carton
R&D partnership portfolios and the inflow of technological knowledge
This article links research on parallel search and joint R&D to contribute a portfolio perspective to the study of knowledge flows within interfirm R&D partnerships. In a longitudinal analysis of firms engaged in R&D partnerships relating to information technology between 1975 and 1999, we show that the size of a firm's R&D partnership portfolio and its share of novel partners both have an inverted U-shaped effect on the inflow of technological knowledge from the firm's R&D partners. We also show how these direct effects vary as a function of the level of technological uncertainty within the portfolio
Hamiltonian walks on Sierpinski and n-simplex fractals
We study Hamiltonian walks (HWs) on Sierpinski and --simplex fractals. Via
numerical analysis of exact recursion relations for the number of HWs we
calculate the connectivity constant and find the asymptotic behaviour
of the number of HWs. Depending on whether or not the polymer collapse
transition is possible on a studied lattice, different scaling relations for
the number of HWs are obtained. These relations are in general different from
the well-known form characteristic of homogeneous lattices which has thus far
been assumed to hold for fractal lattices too.Comment: 22 pages, 6 figures; final versio
Seeing Tree Structure from Vibration
Humans recognize object structure from both their appearance and motion;
often, motion helps to resolve ambiguities in object structure that arise when
we observe object appearance only. There are particular scenarios, however,
where neither appearance nor spatial-temporal motion signals are informative:
occluding twigs may look connected and have almost identical movements, though
they belong to different, possibly disconnected branches. We propose to tackle
this problem through spectrum analysis of motion signals, because vibrations of
disconnected branches, though visually similar, often have distinctive natural
frequencies. We propose a novel formulation of tree structure based on a
physics-based link model, and validate its effectiveness by theoretical
analysis, numerical simulation, and empirical experiments. With this
formulation, we use nonparametric Bayesian inference to reconstruct tree
structure from both spectral vibration signals and appearance cues. Our model
performs well in recognizing hierarchical tree structure from real-world videos
of trees and vessels.Comment: ECCV 2018. The first two authors contributed equally to this work.
Project page: http://tree.csail.mit.edu
Two dimensional self-avoiding walk with hydrogen-like bonding: Phase diagram and critical behaviour
The phase diagram for a two-dimensional self-avoiding walk model on the
square lattice incorporating attractive short-ranged interactions between
parallel sections of walk is derived using numerical transfer matrix
techniques. The model displays a collapse transition. In contrast to the
standard -point model, the transition is first order. The phase diagram
in the full fugacity-temperature plane displays an additional transition line,
when compared to the -point model, as well as a critical transition at
finite temperature in the hamiltonian walk limit.Comment: 22 pages, 13 figures. To appear in Journal of Physics
Predicting solvent accessibility: Higher accuracy using Bayesian statistics and optimized residue substitution classes
We introduce a novel Bayesian probabilistic method for predicting the solvent accessibilities of amino acid residues in globular proteins. Using single sequence data, this method achieves prediction accuracies higher than previously published methods. Substantially improved predictions—comparable to the highest accuracies reported in the literature to date—are obtained by representing alignments of the example proteins and their homologs as strings of residue substitution classes, depending on the side chain types observed at each alignment position. These results demonstrate the applicability of this relatively simple Bayesian approach to structure prediction and illustrate the utility of the classification methodology previously developed to extract information from aligned sets of structurally related proteins. © 1996 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38524/1/4_ftp.pd
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