12,618 research outputs found
Revisiting a result of Ko
In this paper we analyze Ko's Theorem 3.4 in \cite{Ko87}.
We extend point b) of Ko's Theorem by showing that
\poneh(\upcoup)=\upcoup. As a corollary, we get the equality
\ph(\upcoup) = \poneh(\upcoup), which is, to our knowledge, a
unique result of type \poneh({\cal C})=\ph({\cal C}), for a class
that would not be equal to \DP.
With regard to point a) of Ko's Theorem, we observe that it also holds for the
classes \upk{k} and for \fewp.
In spite of this, we prove that point b) of Theorem 3.4 fails for such
classes in a relativized world. This is obtained by showing the
relativized separation of \upcoupk{2} from \poneh(\npconp).
Finally, we suggest a natural line of research arising from these facts
Revisiting a fundamental test of the disc instability model for X-ray binaries
We revisit a core prediction of the disc instability model (DIM) applied to
X-ray binaries. The model predicts the existence of a critical mass transfer
rate, which depends on disc size, separating transient and persistent systems.
We therefore selected a sample of 52 persistent and transient neutron star and
black hole X-ray binaries and verified if observed persistent (transient)
systems do lie in the appropriate stable (unstable) region of parameter space
predicted by the model. We find that, despite the significant uncertainties
inherent to these kinds of studies, the data are in very good agreement with
the theoretical expectations. We then discuss some individual cases that do not
clearly fit into this main conclusion. Finally, we introduce the transientness
parameter as a measure of the activity of a source and show a clear trend of
the average outburst recurrence time to decrease with transientness in
agreement with the DIM predictions. We therefore conclude that, despite
difficulties in reproducing the complex details of the lightcurves, the DIM
succeeds to explain the global behaviour of X-ray binaries averaged over a long
enough period of time.Comment: 12 pages, 4 figures. Accepted for publication in MNRAS. Version 2:
some typos corrected and references adde
Surface energy and boundary layers for a chain of atoms at low temperature
We analyze the surface energy and boundary layers for a chain of atoms at low
temperature for an interaction potential of Lennard-Jones type. The pressure
(stress) is assumed small but positive and bounded away from zero, while the
temperature goes to zero. Our main results are: (1) As at fixed positive pressure , the Gibbs measures and
for infinite chains and semi-infinite chains satisfy path large
deviations principles. The rate functions are bulk and surface energy
functionals and
. The minimizer of the surface functional
corresponds to zero temperature boundary layers. (2) The surface correction to
the Gibbs free energy converges to the zero temperature surface energy,
characterized with the help of the minimum of
. (3) The bulk Gibbs measure and Gibbs
free energy can be approximated by their Gaussian counterparts. (4) Bounds on
the decay of correlations are provided, some of them uniform in
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