**Ergodic theory** (Greek: ἔργον* ergon* "work", ὁδός

- Ergodic transformations
- Examples
- Ergodic theorems
- Probabilistic formulation: Birkhoff–Khinchin theorem
- Mean ergodic theorem
- Convergence of the ergodic means in the Lp norms
- Sojourn time
- Ergodic flows on manifolds
- See also
- References
- Historical references
- Modern references
- External links

Ergodic theory, like probability theory, is based on general notions of measure theory. Its initial development was motivated by problems of statistical physics.

A central concern of ergodic theory is the behavior of a dynamical system when it is allowed to run for a long time. The first result in this direction is the Poincaré recurrence theorem, which claims that almost all points in any subset of the phase space eventually revisit the set. Systems for which the Poincaré recurrence theorem holds are conservative systems; thus all ergodic systems are conservative.

More precise information is provided by various **ergodic theorems** which assert that, under certain conditions, the time average of a function along the trajectories exists almost everywhere and is related to the space average. Two of the most important theorems are those of Birkhoff (1931) and von Neumann which assert the existence of a time average along each trajectory. For the special class of **ergodic systems**, this time average is the same for almost all initial points: statistically speaking, the system that evolves for a long time "forgets" its initial state. Stronger properties, such as mixing and equidistribution, have also been extensively studied.

The problem of metric classification of systems is another important part of the abstract ergodic theory. An outstanding role in ergodic theory and its applications to stochastic processes is played by the various notions of entropy for dynamical systems.

The concepts of ergodicity and the ergodic hypothesis are central to applications of ergodic theory. The underlying idea is that for certain systems the time average of their properties is equal to the average over the entire space. Applications of ergodic theory to other parts of mathematics usually involve establishing ergodicity properties for systems of special kind. In geometry, methods of ergodic theory have been used to study the geodesic flow on Riemannian manifolds, starting with the results of Eberhard Hopf for Riemann surfaces of negative curvature. Markov chains form a common context for applications in probability theory. Ergodic theory has fruitful connections with harmonic analysis, Lie theory (representation theory, lattices in algebraic groups), and number theory (the theory of diophantine approximations, L-functions).

Ergodic theory is often concerned with **ergodic transformations**. The intuition behind such transformations, which act on a given set, is that they do a thorough job "stirring" the elements of that set (e.g., if the set is a quantity of hot oatmeal in a bowl, and if a spoonful of syrup is dropped into the bowl, then iterations of the inverse of an ergodic transformation of the oatmeal will not allow the syrup to remain in a local subregion of the oatmeal, but will distribute the syrup evenly throughout. At the same time, these iterations will not compress or dilate any portion of the oatmeal: they preserve the measure that is density.) Here is the formal definition.

Let *T* : *X*→*X* be a measure-preserving transformation on a measure space (*X*, *Σ*, *μ*), with *μ*(*X*) = 1. Then T is **ergodic** if for every E in Σ with *T*^{−1}(*E*) = *E*, either *μ*(*E*) = 0 or *μ*(*E*) = 1.

- An irrational rotation of the circle
**R**/**Z**,*T*:*x*→*x*+ θ, where θ is irrational, is ergodic. This transformation has even stronger properties of unique ergodicity, minimality, and equidistribution. By contrast, if θ =*p*/*q*is rational (in lowest terms) then*T*is periodic, with period*q*, and thus cannot be ergodic: for any interval*I*of length*a*, 0 <*a*< 1/*q*, its orbit under*T*(that is, the union of*I*,*T*(*I*), ...,*T*^{q−1}(*I*), which contains the image of*I*under any number of applications of*T*) is a*T*-invariant mod 0 set that is a union of*q*intervals of length*a*, hence it has measure*qa*strictly between 0 and 1. - Let
*G*be a compact abelian group,*μ*the normalized Haar measure, and*T*a group automorphism of*G*. Let*G** be the Pontryagin dual group, consisting of the continuous characters of*G*, and*T** be the corresponding adjoint automorphism of*G**. The automorphism*T*is ergodic if and only if the equality (*T**)^{n}(*χ*) =*χ*is possible only when*n*= 0 or*χ*is the trivial character of*G*. In particular, if*G*is the*n*-dimensional torus and the automorphism*T*is represented by a unimodular matrix*A*then*T*is ergodic if and only if no eigenvalue of*A*is a root of unity. - A Bernoulli shift is ergodic. More generally, ergodicity of the shift transformation associated with a sequence of i.i.d. random variables and some more general stationary processes follows from Kolmogorov's zero–one law.
- Ergodicity of a continuous dynamical system means that its trajectories "spread around" the phase space. A system with a compact phase space which has a non-constant first integral cannot be ergodic. This applies, in particular, to Hamiltonian systems with a first integral
*I*functionally independent from the Hamilton function*H*and a compact level set*X*= {(*p*,*q*):*H*(*p*,*q*) = E} of constant energy. Liouville's theorem implies the existence of a finite invariant measure on*X*, but the dynamics of the system is constrained to the level sets of*I*on*X*, hence the system possesses invariant sets of positive but less than full measure. A property of continuous dynamical systems that is the opposite of ergodicity is complete integrability.

Let *T*: *X* → *X* be a measure-preserving transformation on a measure space (*X*, Σ, *μ*) and suppose ƒ is a *μ*-integrable function, i.e. ƒ ∈ *L*^{1}(*μ*). Then we define the following *averages*:

Time average:This is defined as the average (if it exists) over iterations ofTstarting from some initial pointx:

Space average:Ifμ(X) is finite and nonzero, we can consider thespaceorphaseaverage of ƒ:

In general the time average and space average may be different. But if the transformation is ergodic, and the measure is invariant, then the time average is equal to the space average almost everywhere. This is the celebrated ergodic theorem, in an abstract form due to George David Birkhoff. (Actually, Birkhoff's paper considers not the abstract general case but only the case of dynamical systems arising from differential equations on a smooth manifold.) The equidistribution theorem is a special case of the ergodic theorem, dealing specifically with the distribution of probabilities on the unit interval.

More precisely, the **pointwise** or **strong ergodic theorem** states that the limit in the definition of the time average of ƒ exists for almost every *x* and that the (almost everywhere defined) limit function ƒ̂ is integrable:

Furthermore, is *T*-invariant, that is to say

holds almost everywhere, and if *μ*(*X*) is finite, then the normalization is the same:

In particular, if *T* is ergodic, then ƒ̂ must be a constant (almost everywhere), and so one has that

almost everywhere. Joining the first to the last claim and assuming that *μ*(*X*) is finite and nonzero, one has that

for almost all *x*, i.e., for all *x* except for a set of measure zero.

For an ergodic transformation, the time average equals the space average almost surely.

As an example, assume that the measure space (*X*, Σ, *μ*) models the particles of a gas as above, and let ƒ(*x*) denote the velocity of the particle at position *x*. Then the pointwise ergodic theorems says that the average velocity of all particles at some given time is equal to the average velocity of one particle over time.

A generalization of Birkhoff's theorem is Kingman's subadditive ergodic theorem.

**Birkhoff–Khinchin theorem**. Let ƒ be measurable, *E*(|ƒ|) < ∞, and *T* be a measure-preserving map. Then with probability 1:

where is the conditional expectation given the σ-algebra of invariant sets of *T*.

**Corollary** (**Pointwise Ergodic Theorem**): In particular, if *T* is also ergodic, then is the trivial σ-algebra, and thus with probability 1:

**Von Neumann's mean ergodic theorem**, holds in Hilbert spaces.^{ [1] }

Let *U* be a unitary operator on a Hilbert space *H*; more generally, an isometric linear operator (that is, a not necessarily surjective linear operator satisfying ‖*Ux*‖ = ‖*x*‖ for all *x* in *H*, or equivalently, satisfying *U***U* = I, but not necessarily *UU** = I). Let *P* be the orthogonal projection onto {*ψ* ∈ *H* | *Uψ* = ψ} = ker(*I* − *U*).

Then, for any *x* in *H*, we have:

where the limit is with respect to the norm on *H*. In other words, the sequence of averages

converges to *P* in the strong operator topology.

Indeed, it is not difficult to see that in this case any admits an orthogonal decomposition into parts from and respectively. The former part is invariant in all the partial sums as grows, while for the latter part, from the telescoping series one would have:

This theorem specializes to the case in which the Hilbert space *H* consists of *L*^{2} functions on a measure space and *U* is an operator of the form

where *T* is a measure-preserving endomorphism of *X*, thought of in applications as representing a time-step of a discrete dynamical system.^{ [2] } The ergodic theorem then asserts that the average behavior of a function ƒ over sufficiently large time-scales is approximated by the orthogonal component of ƒ which is time-invariant.

In another form of the mean ergodic theorem, let *U _{t}* be a strongly continuous one-parameter group of unitary operators on

converges in the strong operator topology as *T* → ∞. In fact, this result also extends to the case of strongly continuous one-parameter semigroup of contractive operators on a reflexive space.

Remark: Some intuition for the mean ergodic theorem can be developed by considering the case where complex numbers of unit length are regarded as unitary transformations on the complex plane (by left multiplication). If we pick a single complex number of unit length (which we think of as *U*), it is intuitive that its powers will fill up the circle. Since the circle is symmetric around 0, it makes sense that the averages of the powers of *U* will converge to 0. Also, 0 is the only fixed point of *U*, and so the projection onto the space of fixed points must be the zero operator (which agrees with the limit just described).

Let (*X*, Σ, *μ*) be as above a probability space with a measure preserving transformation *T*, and let 1 ≤ *p* ≤ ∞. The conditional expectation with respect to the sub-σ-algebra Σ_{T} of the *T*-invariant sets is a linear projector *E _{T}* of norm 1 of the Banach space

Let (*X*, Σ, *μ*) be a measure space such that *μ*(*X*) is finite and nonzero. The time spent in a measurable set *A* is called the **sojourn time**. An immediate consequence of the ergodic theorem is that, in an ergodic system, the relative measure of *A* is equal to the mean sojourn time:

for all *x* except for a set of measure zero, where χ_{A} is the indicator function of *A*.

The **occurrence times** of a measurable set *A* is defined as the set *k*_{1}, *k*_{2}, *k*_{3}, ..., of times *k* such that *T ^{k}*(

(See almost surely.) That is, the smaller *A* is, the longer it takes to return to it.

The ergodicity of the geodesic flow on compact Riemann surfaces of variable negative curvature and on compact manifolds of constant negative curvature of any dimension was proved by Eberhard Hopf in 1939, although special cases had been studied earlier: see for example, Hadamard's billiards (1898) and Artin billiard (1924). The relation between geodesic flows on Riemann surfaces and one-parameter subgroups on SL(2, **R**) was described in 1952 by S. V. Fomin and I. M. Gelfand. The article on Anosov flows provides an example of ergodic flows on SL(2, **R**) and on Riemann surfaces of negative curvature. Much of the development described there generalizes to hyperbolic manifolds, since they can be viewed as quotients of the hyperbolic space by the action of a lattice in the semisimple Lie group SO(n,1). Ergodicity of the geodesic flow on Riemannian symmetric spaces was demonstrated by F. I. Mautner in 1957. In 1967 D. V. Anosov and Ya. G. Sinai proved ergodicity of the geodesic flow on compact manifolds of variable negative sectional curvature. A simple criterion for the ergodicity of a homogeneous flow on a homogeneous space of a semisimple Lie group was given by Calvin C. Moore in 1966. Many of the theorems and results from this area of study are typical of rigidity theory.

In the 1930s G. A. Hedlund proved that the horocycle flow on a compact hyperbolic surface is minimal and ergodic. Unique ergodicity of the flow was established by Hillel Furstenberg in 1972. Ratner's theorems provide a major generalization of ergodicity for unipotent flows on the homogeneous spaces of the form Γ \ *G*, where *G* is a Lie group and Γ is a lattice in *G*.

In the last 20 years, there have been many works trying to find a measure-classification theorem similar to Ratner's theorems but for diagonalizable actions, motivated by conjectures of Furstenberg and Margulis. An important partial result (solving those conjectures with an extra assumption of positive entropy) was proved by Elon Lindenstrauss, and he was awarded the Fields medal in 2010 for this result.

- Chaos theory
- Ergodic hypothesis
- Ergodic process
- Lyapunov time – the time limit to the predictability of the system
- Maximal ergodic theorem
- Ornstein isomorphism theorem
- Statistical mechanics
- Symbolic dynamics
- Lindy effect

In mathematics, a **dynamical system** is a system in which a function describes the time dependence of a point in a geometrical space. Examples include the mathematical models that describe the swinging of a clock pendulum, the flow of water in a pipe, and the number of fish each springtime in a lake.

In mathematics, a **measure** on a set is a systematic way to assign a number to subsets of a set, intuitively interpreted as the size of the subset. Those sets which can be associated with such a number, we call **measurable sets**. In this sense, a measure is a generalization of the concepts of length, area, and volume. A particularly important example is the Lebesgue measure on a Euclidean space. This assigns the usual length, area, or volume to certain subsets of the given Euclidean space. For instance, the Lebesgue measure of an interval of real numbers is its usual length, but also assigns numbers to other kinds of sets in a way that is consistent with the lengths of intervals.

In mathematical analysis, the **Haar measure** assigns an "invariant volume" to subsets of locally compact topological groups, consequently defining an integral for functions on those groups.

In mathematics, the **Radon–Nikodym theorem** is a result in measure theory that expresses the relationship between two measures defined on the same measurable space. A *measure* is a set function that assigns a consistent magnitude to the measurable subsets of a measurable space. Examples of a measure include area and volume, where the subsets are sets of points; or the probability of an event, which is a subset of possible outcomes within a wider probability space.

In mathematics, a **measure-preserving dynamical system** is an object of study in the abstract formulation of dynamical systems, and ergodic theory in particular. Measure-preserving systems obey the Poincaré recurrence theorem, and are a special case of conservative systems. They provide the formal, mathematical basis for a broad range of physical systems, and, in particular, many systems from classical mechanics as well as systems in thermodynamic equilibrium.

In number theory, Aleksandr Yakovlevich Khinchin proved that for almost all real numbers *x*, coefficients *a*_{i} of the continued fraction expansion of *x* have a finite geometric mean that is independent of the value of *x* and is known as **Khinchin's constant**.

In mathematics, **mixing** is an abstract concept originating from physics: the attempt to describe the irreversible thermodynamic process of mixing in the everyday world: mixing paint, mixing drinks, industrial mixing, *etc*.

The concept of **system of imprimitivity** is used in mathematics, particularly in algebra and analysis, both within the context of the theory of group representations. It was used by George Mackey as the basis for his theory of induced unitary representations of locally compact groups.

In functional analysis, an **abelian von Neumann algebra** is a von Neumann algebra of operators on a Hilbert space in which all elements commute.

In mathematics, the **multiplicative ergodic theorem**, or **Oseledets theorem** provides the theoretical background for computation of Lyapunov exponents of a nonlinear dynamical system. It was proved by Valery Oseledets in 1965 and reported at the International Mathematical Congress in Moscow in 1966. A conceptually different proof of the multiplicative ergodic theorem was found by M. S. Raghunathan. The theorem has been extended to semisimple Lie groups by V. A. Kaimanovich and further generalized in the works of David Ruelle, Grigory Margulis, Anders Karlsson, and François Ledrappier.

In mathematics, the **Bernoulli scheme** or **Bernoulli shift** is a generalization of the Bernoulli process to more than two possible outcomes. Bernoulli schemes appear naturally in symbolic dynamics, and are thus important in the study of dynamical systems. Many important dynamical systems exhibit a repellor that is the product of the Cantor set and a smooth manifold, and the dynamics on the Cantor set are isomorphic to that of the Bernoulli shift. This is essentially the Markov partition. The term *shift* is in reference to the shift operator, which may be used to study Bernoulli schemes. The Ornstein isomorphism theorem shows that Bernoulli shifts are isomorphic when their entropy is equal.

In mathematics, **ergodic flows** occur in geometry, through the geodesic and horocycle flows of closed hyperbolic surfaces. Both of these examples have been understood in terms of the theory of unitary representations of locally compact groups: if Γ is the fundamental group of a closed surface, regarded as a discrete subgroup of the Möbius group G = PSL(2,**R**), then the geodesic and horocycle flow can be identified with the natural actions of the subgroups *A* of real positive diagonal matrices and *N* of lower unitriangular matrices on the unit tangent bundle *G* / Γ. The Ambrose-Kakutani theorem expresses every ergodic flow as the flow built from an invertible ergodic transformation on a measure space using a ceiling function. In the case of geodesic flow, the ergodic transformation can be understood in terms of symbolic dynamics; and in terms of the ergodic actions of Γ on the boundary *S*^{1} = *G* / *AN* and *G* / *A* = *S*^{1} × *S*^{1} \ diag *S*^{1}. Ergodic flows also arise naturally as invariants in the classification of von Neumann algebras: the flow of weights for a factor of type III_{0} is an ergodic flow on a measure space.

In those branches of mathematics called dynamical systems and ergodic theory, the concept of a **wandering set** formalizes a certain idea of movement and mixing in such systems. When a dynamical system has a wandering set of non-zero measure, then the system is a dissipative system. This is very much the opposite of a conservative system, for which the ideas of the Poincaré recurrence theorem apply. Intuitively, the connection between wandering sets and dissipation is easily understood: if a portion of the phase space "wanders away" during normal time-evolution of the system, and is never visited again, then the system is dissipative. The language of wandering sets can be used to give a precise, mathematical definition to the concept of a dissipative system. The notion of wandering sets in phase space was introduced by Birkhoff in 1927.

In mathematics, **ergodicity** expresses the idea that a point of a moving system, either a dynamical system or a stochastic process, will eventually visit all parts of the space that the system moves in, in a uniform and random sense. This implies that the average behavior of the system can be deduced from the trajectory of a "typical" point. Equivalently, a sufficiently large collection of random samples from a process can represent the average statistical properties of the entire process. Ergodicity is a property of the system; it is a statement that the system cannot be reduced or factored into smaller components. Ergodic theory is the study of systems possessing ergodicity.

In mathematics, the **topological entropy** of a topological dynamical system is a nonnegative extended real number that is a measure of the complexity of the system. Topological entropy was first introduced in 1965 by Adler, Konheim and McAndrew. Their definition was modelled after the definition of the Kolmogorov–Sinai, or metric entropy. Later, Dinaburg and Rufus Bowen gave a different, weaker definition reminiscent of the Hausdorff dimension. The second definition clarified the meaning of the topological entropy: for a system given by an iterated function, the topological entropy represents the exponential growth rate of the number of distinguishable orbits of the iterates. An important **variational principle** relates the notions of topological and measure-theoretic entropy.

In mathematics, an **invariant measure** is a measure that is preserved by some function. Ergodic theory is the study of invariant measures in dynamical systems. The Krylov–Bogolyubov theorem proves the existence of invariant measures under certain conditions on the function and space under consideration.

In mathematics, the **Krylov–Bogolyubov theorem** may refer to either of the two related fundamental theorems within the theory of dynamical systems. The theorems guarantee the existence of invariant measures for certain "nice" maps defined on "nice" spaces and were named after Russian-Ukrainian mathematicians and theoretical physicists Nikolay Krylov and Nikolay Bogolyubov who proved the theorems.

In mathematics, a **conservative system** is a dynamical system which stands in contrast to a dissipative system. Roughly speaking, such systems have no friction or other mechanism to dissipate the dynamics, and thus, their phase space does not shrink over time. Precisely speaking, they are those dynamical systems that have a null wandering set: under time evolution, no portion of the phase space ever "wanders away", never to be returned to or revisited. Alternately, conservative systems are those to which the Poincaré recurrence theorem applies. An important special case of conservative systems are the measure-preserving dynamical systems.

In mathematics, **Kingman's subadditive ergodic theorem** is one of several ergodic theorems. It can be seen as a generalization of Birkhoff's ergodic theorem. Intuitively, the subadditive ergodic theorem is a kind of random variable version of Fekete's lemma. As a result, it can be rephrased in the language of probability, e.g. using a sequence of random variables and expected values. The theorem is named after John Kingman.

In the mathematical discipline of ergodic theory, a **Sinai–Ruelle–Bowen (SRB) measure** is an invariant measure that behaves similarly to, but is not an ergodic measure. In order to be ergodic, the time average would need to be equal the space average for almost all initial states , with being the phase space. For an SRB measure , it suffices that the ergodicity condition be valid for initial states in a set of positive Lebesgue measure.

- ↑ Reed, Michael; Simon, Barry (1980).
*Functional Analysis*. Methods of Modern Mathematical Physics.**1**(Rev. ed.). Academic Press. ISBN 0-12-585050-6. - ↑ ( Walters 1982 )

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