arXiv subjects
Publications and source records attributed to Anthony Quas.
Ledrappier and Walters's article "A Relativised Variational Principle for Continuous Transformations", J. Lond. Math. Soc. (2) 16 (1977), no.3, 568-576) is a landmark in the development of Thermodynamic Formalism. This survey, aimed at newcomers to the field and experts in adjacent fields discusses the background, the Ledrappier-Walters article and some subsequent developments in the field.
We study the singular values and Lyapunov exponents of non-stationary random matrix products subject to small, absolutely continuous, additive noise. Consider a fixed sequence of matrices of bounded norm. Independently perturb the matrices by additive noise distributed according to Lebesgue measure on matrices with norm less than $\epsilon$. Then the gaps between the logarithms of the singular values of the random product of $n$ of these matrices are all of order at least $\epsilon^2n$, both in expectation; and almost surely for large $n$. To prove this, we develop recent work of Gorodetski and Kleptsyn \cite{gorodetski2023nonstationary}. That paper gives a very flexible method, based on relative entropy, for showing that a non-stationary product of matrices in SL(d,R) has a strictly positive Lyapunov exponent. We extend their work in two ways, firstly by making the estimates quantitative in the context of absolutely continuous distributions, giving the universal estimates described above; and secondly by developing a fibered version of their methods, working on flag bundles instead of the projective space to estimate gaps between arbitrary consecutive exponents. Our methods retain much of the flexibility of those of Gorodetski and Kleptsyn, and we hope that they will find application in other related problems.
We consider a one-step matrix cocycle generated by a pair of non-negative parabolic matrices and study the equilibrium measures for $t\log \|\mathcal A\|$ as $t$ runs over the reals. We show that there is a freezing first order phase transition at some parameter value $t_c$ so that for $t t_c$, the equilibrium measure is unique, non-atomic and fully supported. The phase transition closely resembles the classical Hofbauer example. In particular, our example shows that there may be non-unique equilibrium measures for negative $t$ even if the cocycle is strongly irreducible and proximal.
We study area preserving Anosov maps on the two-dimensional torus within a fixed homotopy class. We show that the set of pressure functions for Anosov diffeomorphisms with respect to the geometric potential is equal to the set of pressure functions for the linear Anosov automorphism with respect to H\"{o}lder potentials. We use this result to provide a negative answer to the $C^{1+\alpha}$ version of the question posed by Rodriguez Hertz on whether two homotopic area preserving $C^\infty$ Anosov difeomorphisms whose geometric potentials have identical pressure functions must be $C^\infty$ conjugate.
The BCZ map was introduced in 2001 by Boca, Cobeli and Zaharescu as a tool to study the statistical properties of Farey sequences, whose relation to Riemann Hypothesis dates back to Franel and Landau. Later, J. Athreya and the first author observed that the BCZ map arises as a Poincare section of horocycle flow, establishing both ergodicity as well as zero measure-theoretic entropy. In this article, we prove that the BCZ map is weakly mixing, answering the last remaining question about the BCZ map raised in a 2006 survey by Boca and Zaharescu. The proof uses a self-similarity property of the BCZ map that derives from a well-known fact that horocycle flow is renormalized by the geodesic flow, a property already observed in arXiv:1206.6597. We note that the questions of mixing and rigidity remain open.
We study the quantitative simplicity of the Lyapunov spectrum of $d$-dimensional bounded matrix cocycles subjected to additive random perturbations. In dimensions 2 and 3, we establish explicit lower bounds on the gaps between consecutive Lyapunov exponents of the perturbed cocycle, depending only on the scale of the perturbation. In arbitrary dimensions, we show existence of a universal lower bound on these gaps. A novelty of this work is that the bounds provided are uniform over all choices of the original sequence of matrices. Furthermore, we make no stationarity assumptions on this sequence. Hence, our results apply to random and sequential dynamical systems alike.
We consider products of matrices of the form $A_n=O_n+\epsilon N_n$ where $O_n$ is a sequence of $d\times d$ orthogonal matrices and $N_n$ has independent standard normal entries and the $(N_n)$ are mutually independent. We study the Lyapunov exponents of the cocycle as a function of $\epsilon$, giving an exact expression for the $j$th Lyapunov exponent in terms of the Gram-Schmidt orthogonalization of $I+\epsilon N$. Further, we study the asymptotics of these exponents, showing that $\lambda_j=(d-2j)\epsilon^2/2+O(\epsilon^4|\log\epsilon|^4)$.
We study the behavior of the pressure function for H\"{o}lder continuous potentials on mixing subshifts of finite type. The classical theory of thermodynamic formalism shows that such pressure functions are convex, analytic and have slant asymptotes. We provide a sharp exponential lower bound on how fast the pressure function approaches its asymptotes. As a counterpart, we also show that there is no corresponding upper bound by exhibiting systems for which the convergence is arbitrarily slow. However, we prove that the exponential upper bound still holds for a generic H\"{o}lder potential. In addition, we determine that the pressure function satisfies a coarse uniform convexity property. Asymptotic bounds and quantitative convexity estimates are the first additional general properties of the pressure function obtained in the settings of Bowen and Ruelle since their groundbreaking work more than 40 years ago.
We study the flexibility of the pressure function of a continuous potential (observable) with respect to a parameter regarded as the inverse temperature. The points of non-differentiability of this function are of particular interest in statistical physics, since they correspond to phase transitions. It is well known that the pressure function is convex, Lipschitz, and has an asymptote at infinity. We prove that in a setting of one-dimensional compact symbolic systems these are the only restrictions. We present a method to explicitly construct a continuous potential whose pressure function coincides with \emph{any} prescribed convex Lipschitz asymptotically linear function starting at a given positive value of the parameter. In fact, we establish a multidimensional version of this result. As a consequence, we obtain that for a continuous observable the phase transitions can occur at a countable dense set of temperature values. We go further and show that one can vary the cardinality of the set of ergodic equilibrium states as a function of the parameter to be any number, finite or infinite.
We introduce a non-standard model for percolation on the integer lattice $\mathbb Z^2$. Randomly assign to each vertex $a \in \mathbb Z^2$ a potential, denoted $\phi_a$, chosen independently and uniformly from the interval $[0, 1]$. For fixed $\epsilon \in [0,1]$, draw a directed edge from vertex $a$ to a nearest-neighbor vertex $b$ if $\phi_b < \phi_a + \epsilon$, yielding a directed subgraph of the infinite directed graph $\overrightarrow{G}$ whose vertex set is $\mathbb Z^2$, with nearest-neighbor edge set. We define notions of weak and strong percolation for our model, and observe that when $\epsilon = 0$ the model fails to percolate weakly, while for $\epsilon = 1$ it percolates strongly. We show that there is a positive $\epsilon_0$ so that for $0 \le \epsilon \le \epsilon_0$, the model fails to percolate weakly, and that when $\epsilon > p_\text{site}$, the critical probability for standard site percolation in $\mathbb Z^2$, the model percolates strongly. We study the number of infinite strongly connected clusters occurring in a typical configuration. We show that for these models of percolation on directed graphs, there are some subtle issues that do not arise for undirected percolation. Although our model does not have the finite energy property, we are able to show that, as in the standard model, the number of infinite strongly connected clusters is almost surely 0, 1 or $\infty$.
This works investigates the Lyapunov-Oseledets spectrum of transfer operator cocycles associated to one-dimensional random paired tent maps depending on a parameter $\epsilon$, quantifying the strength of the \emph{leakage} between two nearly invariant regions. We show that the system exhibits metastability, and identify the second Lyapunov exponent $\lambda_2^\epsilon$ within an error of order $\epsilon^2|\log \epsilon|$. This approximation agrees with the naive prediction provided by a time-dependent two-state Markov chain. Furthermore, it is shown that $\lambda_1^\epsilon=0$ and $\lambda_2^\epsilon$ are simple, and are the only exceptional Lyapunov exponents of magnitude greater than $-\log2+ O(\log\log\tfrac 1\epsilon/\log\tfrac 1\epsilon)$.
Liverani-Saussol-Vaienti (L-S-V) maps form a family of piecewise differentiable dynamical systems on $[0,1]$ depending on one parameter $\omega\in\mathbb R^+$. These maps are everywhere expanding apart from a neutral fixed point. It is well known that depending on the amount of expansion close to the neutral point, they have either an absolutely continuous invariant probability measure and polynomial decay of correlations ($\omega <1$), or a unique physical measure that is singular and concentrated at the neutral point ($\omega >1$). In this paper, we study the composition of L-S-V maps whose parameters are randomly sampled from a range in $\mathbb R^+$, and where these two contrasting behaviours are mixed. We show that if the parameters $\omega<1$ are sampled with positive probability, then the stationary measure of the random system is absolutely continuous; the annealed decay rate of correlations is close (or in some cases equal) to the fastest rate of decay among those of the sampled systems; and suitably rescaled Birkhoff averages converge to limit laws. In contrast to previous studies where $\omega \in [0,1]$, we allow $ \omega >1$ in our sampling distribution. We also show that one can obtain similar decay of correlation rates for $\omega \in [0,\infty)$, when sampling is done with respect to a family of smooth, heavy-tailed distributions.
Let $\phi:X\to \mathbb R$ be a continuous potential associated with a symbolic dynamical system $T:X\to X$ over a finite alphabet. Introducing a parameter $\beta>0$ (interpreted as the inverse temperature) we study the regularity of the pressure function $\beta\mapsto P_{\rm top}(\beta\phi)$ on an interval $[\alpha,\infty)$ with $\alpha>0$. We say that $\phi$ has a phase transition at $\beta_0$ if the pressure function $P_{\rm top}(\beta\phi)$ is not differentiable at $\beta_0$. This is equivalent to the condition that the potential $\beta_0\phi$ has two (ergodic) equilibrium states with distinct entropies. For any $\alpha>0$ and any increasing sequence of real numbers $(\beta_n)$ contained in $[\alpha,\infty)$, we construct a potential $\phi$ whose phase transitions in $[\alpha,\infty)$ occur precisely at the $\beta_n$'s. In particular, we obtain a potential which has a countably infinite set of phase transitions.
In this article, we continue the structural study of factor maps betweeen symbolic dynamical systems and the relative thermodynamic formalism. Here, one is studying a factor map from a shift of finite type $X$ (equipped with a potential function) to a sofic shift $Z$, equipped with a shift-invariant measure $\nu$. We study relative equilibrium states, that is shift-invariant measures on $X$ that push forward under the factor map to $\nu$ which maximize the relative pressure: the relative entropy plus the integral of $\phi$. In the non-relative case (where $Z$ is the one point shift and the factor map is trivial), these measures have a very broad range of application: in hyperbolic dynamics, information theory, geometry, Teichm\"uller theory and elsewhere). Relative equilibrium states have also been shown to arise naturally in some contexts in geometric measure theory as a description of measures achieving the Hausdorff dimension in ambient spaces. Previous articles have identified relative versions of well-known notions of degree appearing in one-dimensional symbolic settings, and established bounds in terms of these on the number of ergodic relative equilibrium states. In this paper, we establish a new connection to multiplicative ergodic theory by relating these factor triples to a cocycle of Ruelle Perron-Frobenius operators, and showing that the principal Lyapunov exponent of this cocycle is the relative pressure; and the dimension of the leading Oseledets space is equal to the number of measures of relative maximal entropy, counted with a previously-identified concept of multiplicity.
We consider a two-sided sequence of bounded operators in a Banach space which are not necessarily injective and satisfy two properties (SVG) and (FI). The singular value gap (SVG) property says that two successive singular values of the cocycle at some index $d$ admit a uniform exponential gap; the fast invertibility (FI) property says that the cocycle is uniformly invertible on the fastest $d$-dimensional direction. We prove the existence of a uniform equivariant splitting of the Banach space into a fast space of dimension $d$ and a slow space of co-dimension $d$. We compute an explicit constant lower bound on the angle between these two spaces using solely the constants defining the properties (SVG) and (FI). We extend the results obtained in the finite-dimensional case for bijective operators and the results obtained by Blumenthal and Morris in the infinite-dimensional case for injective norm-continuous cocycles, in the direction that the operators are not required to be globally injective, that no dynamical system is involved, and no compactness of the underlying system or smoothness of the cocycle is required. Moreover, we give quantitative estimates of the angle between the fast and slow spaces that are new even in the case of finite-dimensional bijective operators in Hilbert spaces.
In this paper, we study random Blaschke products, acting on the unit circle, and consider the cocycle of Perron-Frobenius operators acting on Banach spaces of analytic functions on an annulus. We completely describe the Lyapunov spectrum of these cocycles. As a corollary, we obtain a simple random Blaschke product system where the Perron-Frobenius cocycle has infinitely many distinct Lyapunov exponents, but where arbitrarily small natural perturbations cause a complete collapse of the Lyapunov spectrum, except for the exponent 0 associated with the absolutely continuous invariant measure. That is, under perturbations, the Lyapunov exponents become 0 with multiplicity 1, and $-\infty$ with infinite multiplicity. This is superficially similar to the finite-dimensional phenomenon, discovered by Bochi \cite{Bochi-thesis}, that away from the uniformly hyperbolic setting, small perturbations can lead to a collapse of the Lyapunov spectrum to zero. In this paper, however, the cocycle and its perturbation are explicitly described; and further, the mechanism for collapse is quite different. We study stability of the Perron-Frobenius cocycles arising from general random Blaschke products. We give a necessary and sufficient criterion for stability of the Lyapunov spectrum in terms of the derivative of the random Blaschke product at its random fixed point, and use this to show that an open dense set of Blaschke product cocycles have hyperbolic Perron-Frobenius cocycles. In the final part, we prove a relationship between the Lyapunov spectrum of a single cocycle acting on two different Banach spaces, allowing us to draw conclusions for the same cocycles acting on $C^r$ functions spaces.
We prove the existence of Rado sets in the Banach space of continuous functions on [0,1]. A countable dense set S is Rado if with probability 1, the infinite geometric random graph on S, formed by probabilistically making adjacent elements of S that are within unit distance of each other, is unique up to isomorphism. We show that for a suitable measure which we construct, almost all countable dense sets in the subspaces of piecewise linear functions and of polynomials are Rado. Moreover, all graphs arising from such sets are of a unique isomorphism type. For the subspace of Brownian motion paths, almost all countable subsets are Rado (for a suitable measure) and the resulting graphs are of a unique isomorphism type. We show that the graph arising from piecewise linear functions and polynomials is not isomorphic to the graph arising from Brownian motion paths. Moreover, these graphs are non-isomorphic to graphs arising from Rado sets in $\mathbb{R}^n$, or the sequence spaces $c$ and $c_0$.
In skew-product systems with contractive factors, all orbits asymptotically approach the graph of the so-called sync function; hence, the corresponding regularity properties primarily matter. In the literature, sync function Lipschitz continuity and differentiability have been proved to hold depending on the derivative of the base reciprocal, if not on its Lyapunov exponent. However, forcing topological features can also impact the sync function regularity. Here, we estimate the total variation of sync functions generated by one-dimensional Markov maps. A sharp condition for bounded variation is obtained depending on parameters, that involves the Markov map topological entropy. The results are illustrated with examples.