31,694 research outputs found
Convergence to Equilibrium of Logit Dynamics for Strategic Games
We present the first general bounds on the mixing time of the Markov chain
associated to the logit dynamics for wide classes of strategic games. The logit
dynamics with inverse noise beta describes the behavior of a complex system
whose individual components act selfishly and keep responding according to some
partial ("noisy") knowledge of the system, where the capacity of the agent to
know the system and compute her best move is measured by the inverse of the
parameter beta.
In particular, we prove nearly tight bounds for potential games and games
with dominant strategies. Our results show that, for potential games, the
mixing time is upper and lower bounded by an exponential in the inverse of the
noise and in the maximum potential difference. Instead, for games with dominant
strategies, the mixing time cannot grow arbitrarily with the inverse of the
noise.
Finally, we refine our analysis for a subclass of potential games called
graphical coordination games, a class of games that have been previously
studied in Physics and, more recently, in Computer Science in the context of
diffusion of new technologies. We give evidence that the mixing time of the
logit dynamics for these games strongly depends on the structure of the
underlying graph. We prove that the mixing time of the logit dynamics for these
games can be upper bounded by a function that is exponential in the cutwidth of
the underlying graph and in the inverse of noise. Moreover, we consider two
specific and popular network topologies, the clique and the ring. For games
played on a clique we prove an almost matching lower bound on the mixing time
of the logit dynamics that is exponential in the inverse of the noise and in
the maximum potential difference, while for games played on a ring we prove
that the time of convergence of the logit dynamics to its stationary
distribution is significantly shorter
Particle-physics constraints on multifractal spacetimes
We study electroweak interactions in the multiscale theory with
-derivatives, a framework where spacetime has the typical features of a
multifractal. In the simplest case with only one characteristic time, length
and energy scale , , and , we consider (i) the muon decay
rate and (ii) the Lamb shift in the hydrogen atom, and constrain the
corrections to the ordinary results. We obtain the independent absolute upper
bounds (i) . Under some
mild theoretical assumptions, the Lamb shift alone yields the even tighter
ranges , , and
. To date, these are the first robust constraints on the
scales at which the multifractal features of the geometry can become important
in a physical process.Comment: 5 pages. v2: units of derived bounds corrected, direct bounds and
conclusions unchanged; v3: minor typos corrected to match the published
versio
Poincar\'e profiles of groups and spaces
We introduce a spectrum of monotone coarse invariants for metric measure
spaces called Poincar\'{e} profiles. The two extremes of this spectrum
determine the growth of the space, and the separation profile as defined by
Benjamini--Schramm--Tim\'{a}r. In this paper we focus on properties of the
Poincar\'{e} profiles of groups with polynomial growth, and of hyperbolic
spaces, where we deduce a connection between these profiles and conformal
dimension. As applications, we use these invariants to show the non-existence
of coarse embeddings in a variety of examples.Comment: 55 pages. To appear in Revista Matem\'atica Iberoamerican
New Standard Model constraints on the scales and dimension of spacetime
Using known estimates for the kaon--antikaon transitions, the mean lifetime
of the muon and the mean lifetime of the tau, we place new and stronger
constraints on the scales of the multi-fractional theories with weighted and
-derivatives. These scenarios reproduce a quantum-gravity regime where
fields live on a continuous spacetime with a scale-dependent Hausdorff
dimension. In the case with weighted derivatives, constraints from the muon
lifetime are various orders of magnitude stronger than those from the tau
lifetime and the kaon--antikaon transitions. The characteristic energy scale of
the theory cannot be greater than , and is
tightened to for the typical value
of the fractional exponents in the spacetime measure. We also find an upper
bound on the spacetime Hausdorff dimension in the ultraviolet.
In the case with -derivatives, the strongest bound comes from the tau
lifetime, but it is about 10 orders of magnitude weaker than for the theory
with weighted derivatives.Comment: 1+21 pages, 3 figures, 1 table. v2: reference adde
Multi-wavelength signals of dark matter annihilations at the Galactic center
We perform a systematic study of the multi-wavelength signal induced by
weakly interacting massive particle (WIMP) annihilations at the Galactic Center
(GC). Referring to a generic WIMP dark matter (DM) scenario and depending on
astrophysical inputs, we discuss spectral and angular features and sketch
correlations among signals in the different energy bands. None of the
components which have been associated to the GC source Sgr A*, nor the diffuse
emission components from the GC region, have spectral or angular features
typical of a DM source. Still, data-sets at all energy bands, namely, the
radio, near infrared, X-ray and gamma-ray bands, contribute to place
significant constraints on the WIMP parameter space. In general, the gamma-ray
energy range is not the one with the largest signal to background ratio. In the
case of large magnetic fields close to the GC, X-ray data give the tightest
bounds. The emission in the radio-band, which is less model dependent, is very
constraining as well. The recent detection by HESS of a GC gamma-ray source,
and of a diffuse gamma-ray component, limits the possibility of a DM discovery
with next generation of gamma-ray telescopes, like GLAST and CTA. We find that
the most of the region in the parameter space accessible to these instruments
is actually already excluded at other wave-lenghts. On the other hand, there
may be still an open window to improve constraints with wide-field radio
observations.Comment: 26 pages, 32 figures, treatments of starlight and interstellar medium
improved, other minor changes, references adde
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