2,595 research outputs found
Moving constraints as stabilizing controls in classical mechanics
The paper analyzes a Lagrangian system which is controlled by directly
assigning some of the coordinates as functions of time, by means of
frictionless constraints. In a natural system of coordinates, the equations of
motions contain terms which are linear or quadratic w.r.t.time derivatives of
the control functions. After reviewing the basic equations, we explain the
significance of the quadratic terms, related to geodesics orthogonal to a given
foliation. We then study the problem of stabilization of the system to a given
point, by means of oscillating controls. This problem is first reduced to the
weak stability for a related convex-valued differential inclusion, then studied
by Lyapunov functions methods. In the last sections, we illustrate the results
by means of various mechanical examples.Comment: 52 pages, 4 figure
The boundary Riemann solver coming from the real vanishing viscosity approximation
We study a family of initial boundary value problems associated to mixed
hyperbolic-parabolic systems:
v^{\epsilon} _t + A (v^{\epsilon}, \epsilon v^{\epsilon}_x ) v^{\epsilon}_x =
\epsilon B (v^{\epsilon} ) v^{\epsilon}_{xx}
The conservative case is, in particular, included in the previous
formulation.
We suppose that the solutions to these problems converge to a
unique limit. Also, it is assumed smallness of the total variation and other
technical hypotheses and it is provided a complete characterization of the
limit.
The most interesting points are the following two.
First, the boundary characteristic case is considered, i.e. one eigenvalue of
can be .
Second, we take into account the possibility that is not invertible. To
deal with this case, we take as hypotheses conditions that were introduced by
Kawashima and Shizuta relying on physically meaningful examples. We also
introduce a new condition of block linear degeneracy. We prove that, if it is
not satisfied, then pathological behaviours may occur.Comment: 84 pages, 6 figures. Text changes in Sections 1 and 3.2.3. Added
Section 3.1.2. Minor changes in other section
Spectro-photometric Evolution of Elliptical Galaxies. II. Models with infall
In this paper we present new chemo-spectro-photometric models of elliptical
galaxies in which infall of primordial gas is allowed to occur. They aim to
simulate the collapse of a galaxy made of two components, i.e. luminous
material and dark matter. The mass of the dark component is assumed to be
constant in time, whereas that of the luminous material is supposed to accrete
at a suitable rate. They also include the effect of galactic winds powered by
supernova explosions and stellar winds from massive, early-type stars. The
models are constrained to match a number of properties of elliptical galaxies,
i.e. the slope and mean colours of the colour-magnitude relation (CMR), V
versus (V--K), the UV excess as measured by the colour (1550--V) together with
the overall shape of the integrated spectral energy distribution (ISED) in the
ultraviolet, the relation between the Mg2 index and (1550--V), the mass to blue
luminosity ratio M/Lb as a function of the B luminosity, and finally the
broad-band colours (U--B), (B--V), (V--I), (V--K), etc.Comment: pages 22, 20 postscript figures, 2 external table (tab2_infall using
supertabular.sty and tab5_infall using supertabular1.sty
Some Results on the Boundary Control of Systems of Conservation Laws
This note is concerned with the study of the initial boundary value problem
for systems of conservation laws from the point of view of control theory,
where the initial data is fixed and the boundary data are regarded as control
functions. We first consider the problem of controllability at a fixed time for
genuinely nonlinear Temple class systems, and present a description of the set
of attainable configurations of the corresponding solutions in terms of
suitable Oleinik-type estimates. We next present a result concerning the
asymptotic stabilization near a constant state for general systems.
Finally we show with an example that in general one cannot achieve exact
controllability to a constant state in finite time.Comment: 10 pages, 4 figures, conferenc
Effects of dark matter annihilation on the first stars
We study the evolution of the first stars in the universe (Population III)
from the early pre-Main Sequence until the end of helium burning in the
presence of WIMP dark matter annihilation inside the stellar structure. The two
different mechanisms that can provide this energy source are the contemporary
contraction of baryons and dark matter, and the capture of WIMPs by scattering
off the gas with subsequent accumulation inside the star. We find that the
first mechanism can generate an equilibrium phase, previously known as a "dark
star", which is transient and present in the very early stages of pre-MS
evolution. The mechanism of scattering and capture acts later, and can support
the star virtually forever, depending on environmental characteristic of the
dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star
Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and
R. Schneider ed
Seed treatment with Streptomyces spp. strains inhibit Colletotrichum Sublineolum in sorghum seed.
Suppression of Fusarium moniforme by Streptomyces spp. isolates in relation to dose- response relationship
Suplemento. Edição do IX International Congress of Plant Pathology, 2008, Torino. Abstracts on invited and offered papers
- …