1,017 research outputs found
Random walk on surfaces with hyperbolic cusps
We consider the operator associated to a random walk on finite volume
surfaces with hyperbolic cusps. We study the spectral gap (upper and lower
bound) associated to this operator and deduce some rate of convergence of the
iterated kernel towards its stationary distribution.Comment: 28 page
Cutoff for the Ising model on the lattice
Introduced in 1963, Glauber dynamics is one of the most practiced and
extensively studied methods for sampling the Ising model on lattices. It is
well known that at high temperatures, the time it takes this chain to mix in
on a system of size is . Whether in this regime there is
cutoff, i.e. a sharp transition in the -convergence to equilibrium, is a
fundamental open problem: If so, as conjectured by Peres, it would imply that
mixing occurs abruptly at for some fixed , thus providing
a rigorous stopping rule for this MCMC sampler. However, obtaining the precise
asymptotics of the mixing and proving cutoff can be extremely challenging even
for fairly simple Markov chains. Already for the one-dimensional Ising model,
showing cutoff is a longstanding open problem.
We settle the above by establishing cutoff and its location at the high
temperature regime of the Ising model on the lattice with periodic boundary
conditions. Our results hold for any dimension and at any temperature where
there is strong spatial mixing: For this carries all the way to the
critical temperature. Specifically, for fixed , the continuous-time
Glauber dynamics for the Ising model on with periodic boundary
conditions has cutoff at , where is
the spectral gap of the dynamics on the infinite-volume lattice. To our
knowledge, this is the first time where cutoff is shown for a Markov chain
where even understanding its stationary distribution is limited.
The proof hinges on a new technique for translating to mixing
which enables the application of log-Sobolev inequalities. The technique is
general and carries to other monotone and anti-monotone spin-systems.Comment: 34 pages, 3 figure
Entropy-driven cutoff phenomena
In this paper we present, in the context of Diaconis' paradigm, a general
method to detect the cutoff phenomenon. We use this method to prove cutoff in a
variety of models, some already known and others not yet appeared in
literature, including a chain which is non-reversible w.r.t. its stationary
measure. All the given examples clearly indicate that a drift towards the
opportune quantiles of the stationary measure could be held responsible for
this phenomenon. In the case of birth- and-death chains this mechanism is
fairly well understood; our work is an effort to generalize this picture to
more general systems, such as systems having stationary measure spread over the
whole state space or systems in which the study of the cutoff may not be
reduced to a one-dimensional problem. In those situations the drift may be
looked for by means of a suitable partitioning of the state space into classes;
using a statistical mechanics language it is then possible to set up a kind of
energy-entropy competition between the weight and the size of the classes.
Under the lens of this partitioning one can focus the mentioned drift and prove
cutoff with relative ease.Comment: 40 pages, 1 figur
Cutoff for the East process
The East process is a 1D kinetically constrained interacting particle system,
introduced in the physics literature in the early 90's to model liquid-glass
transitions. Spectral gap estimates of Aldous and Diaconis in 2002 imply that
its mixing time on sites has order . We complement that result and show
cutoff with an -window.
The main ingredient is an analysis of the front of the process (its rightmost
zero in the setup where zeros facilitate updates to their right). One expects
the front to advance as a biased random walk, whose normal fluctuations would
imply cutoff with an -window. The law of the process behind the
front plays a crucial role: Blondel showed that it converges to an invariant
measure , on which very little is known. Here we obtain quantitative
bounds on the speed of convergence to , finding that it is exponentially
fast. We then derive that the increments of the front behave as a stationary
mixing sequence of random variables, and a Stein-method based argument of
Bolthausen ('82) implies a CLT for the location of the front, yielding the
cutoff result.
Finally, we supplement these results by a study of analogous kinetically
constrained models on trees, again establishing cutoff, yet this time with an
-window.Comment: 33 pages, 2 figure
Discrete analogue computing with rotor-routers
Rotor-routing is a procedure for routing tokens through a network that can
implement certain kinds of computation. These computations are inherently
asynchronous (the order in which tokens are routed makes no difference) and
distributed (information is spread throughout the system). It is also possible
to efficiently check that a computation has been carried out correctly in less
time than the computation itself required, provided one has a certificate that
can itself be computed by the rotor-router network. Rotor-router networks can
be viewed as both discrete analogues of continuous linear systems and
deterministic analogues of stochastic processes.Comment: To appear in Chaos Special Focus Issue on Intrinsic and Designed
Computatio
Stein's Method and Characters of Compact Lie Groups
Stein's method is used to study the trace of a random element from a compact
Lie group or symmetric space. Central limit theorems are proved using very
little information: character values on a single element and the decomposition
of the square of the trace into irreducible components. This is illustrated for
Lie groups of classical type and Dyson's circular ensembles. The approach in
this paper will be useful for the study of higher dimensional characters, where
normal approximations need not hold.Comment: 22 pages; same results, but more efficient exposition in Section 3.
Superclasses and supercharacters of normal pattern subgroups of the unipotent upper triangular matrix group
Let denote the group of unipotent upper-triangular matrices
over a fixed finite field \FF_q, and let U_\cP denote the pattern subgroup
of corresponding to the poset \cP. This work examines the superclasses
and supercharacters, as defined by Diaconis and Isaacs, of the family of normal
pattern subgroups of . After classifying all such subgroups, we describe
an indexing set for their superclasses and supercharacters given by set
partitions with some auxiliary data. We go on to establish a canonical
bijection between the supercharacters of U_\cP and certain \FF_q-labeled
subposets of \cP. This bijection generalizes the correspondence identified by
Andr\'e and Yan between the supercharacters of and the \FF_q-labeled
set partitions of . At present, few explicit descriptions appear
in the literature of the superclasses and supercharacters of infinite families
of algebra groups other than \{U_n : n \in \NN\}. This work signficantly
expands the known set of examples in this regard.Comment: 28 page
A quantum de Finetti theorem in phase space representation
The quantum versions of de Finetti's theorem derived so far express the
convergence of n-partite symmetric states, i.e., states that are invariant
under permutations of their n parties, towards probabilistic mixtures of
independent and identically distributed (i.i.d.) states. Unfortunately, these
theorems only hold in finite-dimensional Hilbert spaces, and their direct
generalization to infinite-dimensional Hilbert spaces is known to fail. Here,
we address this problem by considering invariance under orthogonal
transformations in phase space instead of permutations in state space, which
leads to a new type of quantum de Finetti's theorem that is particularly
relevant to continuous-variable systems. Specifically, an n-mode bosonic state
that is invariant with respect to this continuous symmetry in phase space is
proven to converge towards a probabilistic mixture of i.i.d. Gaussian states
(actually, n identical thermal states).Comment: 5 page
Relaxation due to random collisions with a many-qudit environment
We analyze the dynamics of a system qudit of dimension mu sequentially
interacting with the nu-dimensional qudits of a chain playing the ore of an
environment. Each pairwise collision has been modeled as a random unitary
transformation. The relaxation to equilibrium of the purity of the system
qudit, averaged over random collisions, is analytically computed by means of a
Markov chain approach. In particular, we show that the steady state is the one
corresponding to the steady state for random collisions with a single
environment qudit of effective dimension nu_e=nu*mu. Finally, we numerically
investigate aspects of the entanglement dynamics for qubits (mu=nu=2) and show
that random unitary collisions can create multipartite entanglement between the
system qudit and the qudits of the chain.Comment: 7 pages, 6 figure
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