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Alain Thomas

Publications and source records attributed to Alain Thomas.

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Normalized image of a vector by an infinite product of nonnegative matrices

To prove that a measure, linearly representable by means of a finite set of nonnegative matrices $\mathcal M$, has the weak-Gibbs property, one check the uniform convergence (on $\mathcal M^\mathbb N$) of the sequence of vectors $\frac{A_1\cdots A_nc}{\Vert A_1\cdots A_nc\Vert}$ ($c$ positive column-vector). The main theorem gives a sufficient condition for this sequence to converge pointwise. This theorem generalizes the Birkhoff contraction method because it can be used even if the matrices have many zero entries. We also look at the convergence of the sequence of matrices $\frac{A_1\cdots A_n}{\Vert A_1\cdots A_n\Vert}$. The measures defined by Bernoulli convolution are in certain cases linearly representable; we give two example of weak-Gibbs Bernoullt convolutions, by using the Birkhoff contraction coefficient for the first and the theorem for the second. Furthermore we explicit the relationship between the notions of Bernoulli convolution, fundamental curves and lattice two-scale difference equations.

math.FA

Normalized image of a vector by an infinite product of nonnegative matrices

A sofic measure is the image of a Markov probability measure by a continuous morphism, and can be represented by means of products of matrices $A_n$ that belong to a finite set of nonnegative matrices. To prove that the multifractal formalism holds for such a measure, it is necessary to know whenever the sequence $n\mapsto\frac{A_1\cdots A_nv}{\Vert A_1\cdots A_nv\Vert}$ converges when $v$ is a positive vector. We give a sufficient condition for this convergence, that we use for the study of one Bernoulli convolution.

math.FA

A non-uniform distribution property of most orbits, in case the $3x+1$ conjecture is true

Let $T(n)=\left\{\begin{array}{ll}3n+1&(n\hbox{ odd})\frac n2&(n\hbox{ even})\end{array}\right.$ ($n\in\mathbb Z$). We call "the orbit of the integer $n$", the set $$ \mathcal O_n:=\{m\in\mathbb Z\;:\;\exists k\ge0,\ m=T^k(n)\} $$ and we put $c_i(n):=\#\{m\in\mathcal O_n\;:\;m\equiv i\hbox{ mod.}18\}$. Let $W$ be the set of the integers whose orbit contains $1$ and is, in the following sense, about well distributed modulo $18$ between the six elements of the set $I:=\{1,5,7,11,13,17\}$ (the elements of \{1,\dots,18\} that are odd and not divisible by $3$). More precisely: $$ W:=\Big\{n\in\mathbb N\;:\;\exists k\ge0,\ T^k(n)=1\hbox{ and }\forall i\in I,\ \frac{c_i(n)}{\sum_{i\in I}c_i(n)}\le\frac16+0.0215\Big\}. $$ We prove that $W$ has density $0$ in $\mathbb N$. Consequently, if the $3x+1$ conjecture is true, most of the positive integers $n$ satisfy $$ \frac{\max_{i\in I}c_i(n)}{\sum_{i\in I}c_i(n)}>\frac16+0.0215. $$

math.NT

How to prove that some Bernoulli convolution has the weak Gibbs property

In this paper we give an example of uniform convergence of the sequence of column vectors $\displaystyle{A_1\dots A_nV\over\left\Vert A_1\dots A_nV\right\Vert}$, $A_i\in\{A,B,C\}$, $A,B,C$ being some $(0,1)$-matrices of order $7$ with much null entries, and $V$ a fixed positive column vector. These matrices come from the study of the Bernoulli convolution in the base $β>1$ such that $β^3=2β^2-β+1$, that is, the (continuous singular) probability distribution of the random variable $\displaystyle(β-1)\sum_{n=1}^\infty{ω_n\overβ^n}$ when the independent random variables $ω_n$ take the values $0$ and $1$ with probability $\displaystyle{1\over2}$. In the last section we deduce, from the uniform convergence of $\displaystyle{A_1\dots A_nV\over\left\Vert A_1\dots A_nV\right\Vert}$, the Gibbs and the multifractal properties of this measure.

math.DS

Projective convergence of columns for inhomogeneous products of matrices with nonnegative entries

Let $P_n$ be the $n$-step right product $A_1\cdots A_n$, where $A_1,A_2,\dots$ is a given infinite sequence of $d\times d$ matrices with nonnegative entries. In a wide range of situations, the normalized matrix product $P_n/{\Vert P_n\Vert}$ does not converge and we shall be rather interested in the asymptotic behavior of the normalized columns $P_nU_i/\Vert P_nU_i\Vert$, where $U_1,\dots,U_d$ are the canonical $d\times 1$ vectors. Our main result in Theorem~A gives a sufficient condition ${\bf (C)}$ over the sequence $A_1,A_2,\dots$ ensuring the existence of {\it dominant columns} of $P_n$, having the same projective limit $V$: more precisely, for any rank $n$, there exists a partition of $\{1,\dots,d\}$ made of two subsets $J_n\ne\emptyset$ and $J_n^c$ such that each one of the sequences of normalized columns, say $P_nU_{j_n}/\Vert P_nU_{j_n}\Vert$ with $j_n\in J_n$ tends to $V$ as $n$ tends to $+\infty$ and are {\it dominant} in the sense that the ratio $\Vert P_nU_{j_n'}/\Vert P_nU_{j_n}\Vert$ tends to $0$, as soon as $j_n'\in J_n^c$. The existence of sequences of such {\it dominant columns} implies that for any probability vector $X$ with positive entries, the probability vector $P_nX/\Vert P_nX\Vert$, converges as $n$ tends to $+\infty$. Our main application of Theorem~A (and our initial motivation) is related to an {\it Erd\H os problem} concerned with a family of probability measures $μ_β$ (for $1<β<2$ a real parameter) fully supported by a subinterval of the real line, known as {\it Bernoulli convolutions}.

math.PR

Sofic measures and densities of level sets

The Bernoulli convolution associated to the real $β>1$ and the probability vector $(p_0,..,p_{d-1})$ is a probability measure $η_{β,p}$ on $\mathbb R$, solution of the self-similarity relation $\displaystyleη=\sum_{k=0}^{d-1}p_k\cdotη\circ S_k$ where $S_k(x)=\frac{x+k}β$. If $β$ is an integer or a Pisot algebraic number with finite Rényi expansion, $η_{β,p}$ is sofic and a Markov chain is naturally associated. If $β=b\in\mathbb N$ and $p_0=...=p_{d-1}=\frac1d$, the study of $η_{b,p}$ is close to the study of the order of growth of the number of representations in base $b$ with digits in $\{0,1,..,d-1\}$. In the case $b=2$ and $d=3$ it has also something to do with the metric properties of the continued fractions.

math.DS

Almost sure convergence of products of $2\times2$ nonnegative matrices

We study the almost sure convergence of the normalized columns in an infinite product of nonnegative matrices, and the almost sure rank one property of its limit points. Given a probability on the set of $2\times2$ nonnegative matrices, with finite support $\mathcal A=\{A(0),\dots,A(s-1)\}$, and assuming that at least one of the $A(k)$ is not diagonal, the normalized columns of the product matrix $P_n=A(ω_1)\dots A(ω_n)$ converge almost surely (for the product probability) with an exponential rate of convergence if and only if the Lyapunov exponents are almost surely distinct. If this condition is satisfied, given a nonnegative column vector $V$ the column vector $\frac{P_nV}{\Vert P_nV\Vert}$ also converges almost surely with an exponential rate of convergence. On the other hand if we assume only that at least one of the $A(k)$ do not have the form $\begin{pmatrix}a&0\\0&d\end{pmatrix}$, $ad\ne0$, nor the form $\begin{pmatrix}0&b\\d&0\end{pmatrix}$, $bc\ne0$, the limit-points of the normalized product matrix $\frac{P_n}{\Vert P_n\Vert}$ have almost surely rank 1 -although the limits of the normalized columns can be distinct- and $\frac{P_nV}{\Vert P_nV\Vert}$ converges almost surely with a rate of convergence that can be exponential or not exponential.

math.PR

Infinite products of nonnegative $2\times2$ matrices by nonnegative vectors

Given a finite set $\{M_0,\dots,M_{d-1}\}$ of nonnegative $2\times 2$ matrices and a nonnegative column-vector $V$, we associate to each $(ω_n)\in\{0,\dots,d-1\}^\mathbb N$ the sequence of the column-vectors $\displaystyle{M_{ω_1}\dots M_{ω_n}V\over\Vert M_{ω_1}\dots M_{ω_n}V\Vert}$. We give the necessary and sufficient condition on the matrices $M_k$ and the vector $V$ for this sequence to converge for all \hbox{$(ω_n)\in\{0,\dots,d-1\}^\mathbb N$} such that $\forall n,\ M_{ω_1}\dots M_{ω_n}V\ne\begin{pmatrix}0\\0\end{pmatrix}$.

math.RA

Can an infinite left-product of nonnegative matrices be expressed in terms of infinite left-products of stochastic ones?

If a left-product $M_n... M_1$ of square complex matrices converges to a nonnull limit when $n\to\infty$ and if the $M_n$ belong to a finite set, it is clear that there exists an integer $n_0$ such that the $M_n$, $n\ge n_0$, have a common right-eigenvector $V$ for the eigenvalue 1. Now suppose that the $M_n$ are nonnegative and that $V$ has positive entries. Denoting by $Δ$ the diagonal matrix whose diagonal entries are the entries of $V$, the stochastic matrices $S_n=Δ^{-1}M_nΔ$ satisfy $M_n... M_{n_0}=ΔS_n... S_{n_0}Δ^{-1}$, so the problem of the convergence of $M_n... M_1$ reduces to the one of $S_n... S_{n_0}$. In this paper we still suppose that the $M_n$ are nonnegative but we do not suppose that $V$ has positive entries. The first section details the case of the $2\times2$ matrices, and the last gives a first approach in the case of $d\times d$ matrices.

math.PR

Infinite products of $2\times2$ matrices and the Gibbs properties of Bernoulli convolutions

We consider the infinite sequences $(A\_n)\_{n\in\NN}$ of $2\times2$ matrices with nonnegative entries, where the $A\_n$ are taken in a finite set of matrices. Given a vector $V=\pmatrix{v\_1\cr v\_2}$ with $v\_1,v\_2>0$, we give a necessary and sufficient condition for $\displaystyle{A\_1... A\_nV\over|| A\_1... A\_nV||}$ to converge uniformly. In application we prove that the Bernoulli convolutions related to the numeration in Pisot quadratic bases are weak Gibbs.

math.NT

Weak Gibbs property and system of numeration

We study the selfsimilarity and the Gibbs properties of several measures defined on the product space $Ω\_r:=\{0,1,...,\break r-1\}^{\mathbb N}$. This space can be identified with the interval $[0,1]$ by means of the numeration in base $r$. The last section is devoted to the Bernoulli convolution in base $β={1+\sqrt5\over2}$, called the Erd\H os measure, and its analogue in base $-β=-{1+\sqrt5\over2}$, that we study by means of a suitable system of numeration.

math.NT

On the Gibbs properties of Bernoulli convolutions related to $\beta$-numeration in multinacci bases

We consider infinitely convolved Bernoulli measures (or simply Bernoulli convolutions) related to the $\beta$-numeration. A matrix decomposition of these measures is obtained in the case when $\beta$ is a PV number. We also determine their Gibbs properties for $\beta$ being a multinacci number, which makes the multifractal analysis of the corresponding Bernoulli convolution possible.

math.NT