1,431 research outputs found
An Algebra of Pieces of Space -- Hermann Grassmann to Gian Carlo Rota
We sketch the outlines of Gian Carlo Rota's interaction with the ideas that
Hermann Grassmann developed in his Ausdehnungslehre of 1844 and 1862, as
adapted and explained by Giuseppe Peano in 1888. This leads us past what Rota
variously called 'Grassmann-Cayley algebra', or 'Peano spaces', to the Whitney
algebra of a matroid, and finally to a resolution of the question "What,
really, was Grassmann's regressive product?". This final question is the
subject of ongoing joint work with Andrea Brini, Francesco Regonati, and
William Schmitt.
The present paper was presented at the conference "The Digital Footprint of
Gian-Carlo Rota: Marbles, Boxes and Philosophy" in Milano on 17 Feb 2009. It
will appear in proceedings of that conference, to be published by Springer
Verlag.Comment: 28 page
The linear mirror for solar energy exploitation
We describe a simple two-dimensional array of plane mirrors operated by only two motors, which collects efficiently Sun light in order to produce electrical power at about the cost of oil. The system preserves the merits of previous
state-of-the-art solar power plants but is simpler and by far less expensive. A first prototype has been operated at the Physics Department of the University of Udine providing a power of 0.56 kW per m2 of mirror surface in mid November
On-the-fly Uniformization of Time-Inhomogeneous Infinite Markov Population Models
This paper presents an on-the-fly uniformization technique for the analysis
of time-inhomogeneous Markov population models. This technique is applicable to
models with infinite state spaces and unbounded rates, which are, for instance,
encountered in the realm of biochemical reaction networks. To deal with the
infinite state space, we dynamically maintain a finite subset of the states
where most of the probability mass is located. This approach yields an
underapproximation of the original, infinite system. We present experimental
results to show the applicability of our technique
Transition probabilities for general birth-death processes with applications in ecology, genetics, and evolution
A birth-death process is a continuous-time Markov chain that counts the
number of particles in a system over time. In the general process with
current particles, a new particle is born with instantaneous rate
and a particle dies with instantaneous rate . Currently no robust and
efficient method exists to evaluate the finite-time transition probabilities in
a general birth-death process with arbitrary birth and death rates. In this
paper, we first revisit the theory of continued fractions to obtain expressions
for the Laplace transforms of these transition probabilities and make explicit
an important derivation connecting transition probabilities and continued
fractions. We then develop an efficient algorithm for computing these
probabilities that analyzes the error associated with approximations in the
method. We demonstrate that this error-controlled method agrees with known
solutions and outperforms previous approaches to computing these probabilities.
Finally, we apply our novel method to several important problems in ecology,
evolution, and genetics
Design and construction of new central and forward muon counters for CDF II
New scintillation counters have been designed and constructed for the CDF
upgrade in order to complete the muon coverage of the central CDF detector, and
to extend this coverage to larger pseudorapidity. A novel light collection
technique using wavelength shifting fibers, together with high quality
polystyrene-based scintillator resulted in compact counters with good and
stable light collection efficiency over lengths extending up to 320 cm. Their
design and construction is described and results of their initial performance
are reported.Comment: 20 pages, 15 figure
Impact of targeting Kt instead of Kt/V
Producción CientíficaBackground. Patients must receive an adequate dialysis dose in each hemodialysis (HD) session. Ionic dialysance (ID) enables the dialysis dose to be monitored in each session. The aim of this study was to compare the achievement of Kt versus eKt/V values and to analyse the main impediments to reaching
the dialysis dose. Methods. Of 5316 patients from 54 Fresenius Medical Care centers in Spain undergoing their usual HD regime, 3275 received ID and were included in the study.
Results. The minimum prescribed dose of eKt/V was reached in 91.2% of the patients, while the minimum recommended dose of Kt was reached in only 66.8%. Patients not receiving the minimum Kt dose were older, had spent 7 months less on dialysis, had a dialysis duration of 6 min less, had 5.7 kg more
of body weight and Qb was 47 mL/min lower. The target Kt was not reached by 62% of patients with catheters and by 37% of women. With each quintile increase of body weight, eKt/V decreased and Kt increased. Of patients with a body weight >80 kg, 1.4%, mostly men, reached the target Kt but not prescribed eKt/V. Conclusions. The impact of monitoring the dose with Kt instead of Kt/V is that identifies 25.8% of patients who did not reach the minimum Kt while achieving Kt/V. The main impediments
to achieving an adequate dialysis dose were catheter use, female sex, advanced age, greater body weight, shorter dialysis time and lower Qb
Spectral properties of zero temperature dynamics in a model of a compacting granular column
The compacting of a column of grains has been studied using a one-dimensional
Ising model with long range directed interactions in which down and up spins
represent orientations of the grain having or not having an associated void.
When the column is not shaken (zero 'temperature') the motion becomes highly
constrained and under most circumstances we find that the generator of the
stochastic dynamics assumes an unusual form: many eigenvalues become
degenerate, but the associated multi-dimensional invariant spaces have but a
single eigenvector. There is no spectral expansion and a Jordan form must be
used. Many properties of the dynamics are established here analytically; some
are not. General issues associated with the Jordan form are also taken up.Comment: 34 pages, 4 figures, 3 table
Precise Critical Exponents for the Basic Contact Process
We calculated some of the critical exponents of the directed percolation
universality class through exact numerical diagonalisations of the master
operator of the one-dimensional basic contact process. Perusal of the power
method together with finite-size scaling allowed us to achieve a high degree of
accuracy in our estimates with relatively little computational effort. A simple
reasoning leading to the appropriate choice of the microscopic time scale for
time-dependent simulations of Markov chains within the so called quantum chain
formulation is discussed. Our approach is applicable to any stochastic process
with a finite number of absorbing states.Comment: LaTeX 2.09, 9 pages, 1 figur
Ethyl 1-(2-hydroxyethyl)-2-p-tolyl-1H-benzimidazole-5-carboxylate
The asymmetric unit of the title compound, C19H20N2O3, contains two molecules (A and B) with slightly different orientations of the ethyl groups with respect to the attached carboxylate groups. Intramolecular C—H⋯O hydrogen bonds generate S(8) ring motifs in both molecules A and B. In each molecule, the benzimidazole ring system is essentially planar, with maximum deviations of 0.023 (1) and 0.020 (1) Å, respectively, for molecules A and B. The dihedral angle between the benzimidazole ring system and the phenyl ring is 37.34 (5)° for molecule A and 42.42 (5)° for molecule B. In the crystal, O—H⋯N and C—H⋯O hydrogen bonds link the molecules into [100] columns with a cross-section of two-molecule by two-molecule wide, and further stabilization is provided by weak C—H⋯π and π–π interactions [centroid separations = 3.5207 (7) and 3.6314 (8) Å]
Устройство для перемещения датчиков в магнитном поле малогабаритного бетатрона
Рассматривается возможность увеличения точности измерений характеристик магнитного поля посредством более точной установки датчиков в исследуемой точке
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