# mathlib3documentation

dynamics.ergodic.conservative

# Conservative systems #

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In this file we define f : α → α to be a conservative system w.r.t a measure μ if f is non-singular (measure_theory.quasi_measure_preserving) and for every measurable set s of positive measure at least one point x ∈ s returns back to s after some number of iterations of f. There are several properties that look like they are stronger than this one but actually follow from it:

• measure_theory.conservative.frequently_measure_inter_ne_zero, measure_theory.conservative.exists_gt_measure_inter_ne_zero: if μ s ≠ 0, then for infinitely many n, the measure of s ∩ (f^[n]) ⁻¹' s is positive.

• measure_theory.conservative.measure_mem_forall_ge_image_not_mem_eq_zero, measure_theory.conservative.ae_mem_imp_frequently_image_mem: a.e. every point of s visits s infinitely many times (Poincaré recurrence theorem).

We also prove the topological Poincaré recurrence theorem measure_theory.conservative.ae_frequently_mem_of_mem_nhds. Let f : α → α be a conservative dynamical system on a topological space with second countable topology and measurable open sets. Then almost every point x : α is recurrent: it visits every neighborhood s ∈ 𝓝 x infinitely many times.

## Tags #

conservative dynamical system, Poincare recurrence theorem

structure measure_theory.conservative {α : Type u_2} (f : α α) (μ : . "volume_tac") :
Prop

We say that a non-singular (measure_theory.quasi_measure_preserving) self-map is conservative if for any measurable set s of positive measure there exists x ∈ s such that x returns back to s under some iteration of f.

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theorem measure_theory.measure_preserving.conservative {α : Type u_2} {f : α α} {μ : measure_theory.measure α} (h : μ) :

A self-map preserving a finite measure is conservative.

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The identity map is conservative w.r.t. any measure.

theorem measure_theory.conservative.frequently_measure_inter_ne_zero {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) (h0 : μ s 0) :

If f is a conservative map and s is a measurable set of nonzero measure, then for infinitely many values of m a positive measure of points x ∈ s returns back to s after m iterations of f.

theorem measure_theory.conservative.exists_gt_measure_inter_ne_zero {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) (h0 : μ s 0) (N : ) :
(m : ) (H : m > N), μ (s f^[m] ⁻¹' s) 0

If f is a conservative map and s is a measurable set of nonzero measure, then for an arbitrarily large m a positive measure of points x ∈ s returns back to s after m iterations of f.

theorem measure_theory.conservative.measure_mem_forall_ge_image_not_mem_eq_zero {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) (n : ) :
μ {x ∈ s | (m : ), m n f^[m] x s} = 0

Poincaré recurrence theorem: given a conservative map f and a measurable set s, the set of points x ∈ s such that x does not return to s after ≥ n iterations has measure zero.

theorem measure_theory.conservative.ae_mem_imp_frequently_image_mem {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) :
∀ᵐ (x : α) μ, x s (∃ᶠ (n : ) in filter.at_top, f^[n] x s)

Poincaré recurrence theorem: given a conservative map f and a measurable set s, almost every point x ∈ s returns back to s infinitely many times.

theorem measure_theory.conservative.inter_frequently_image_mem_ae_eq {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) :
s {x : α | ∃ᶠ (n : ) in filter.at_top, f^[n] x s} =ᵐ[μ] s
theorem measure_theory.conservative.measure_inter_frequently_image_mem_eq {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) :
μ (s {x : α | ∃ᶠ (n : ) in filter.at_top, f^[n] x s}) = μ s
theorem measure_theory.conservative.ae_forall_image_mem_imp_frequently_image_mem {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) :
∀ᵐ (x : α) μ, (k : ), f^[k] x s (∃ᶠ (n : ) in filter.at_top, f^[n] x s)

Poincaré recurrence theorem: if f is a conservative dynamical system and s is a measurable set, then for μ-a.e. x, if the orbit of x visits s at least once, then it visits s infinitely many times.

theorem measure_theory.conservative.frequently_ae_mem_and_frequently_image_mem {α : Type u_2} {f : α α} {s : set α} {μ : measure_theory.measure α} (hf : μ) (hs : measurable_set s) (h0 : μ s 0) :
∃ᵐ (x : α) μ, x s ∃ᶠ (n : ) in filter.at_top, f^[n] x s

If f is a conservative self-map and s is a measurable set of positive measure, then μ.ae-frequently we have x ∈ s and s returns to s under infinitely many iterations of f.

theorem measure_theory.conservative.ae_frequently_mem_of_mem_nhds {α : Type u_2} {f : α α} {μ : measure_theory.measure α} (h : μ) :
∀ᵐ (x : α) μ, (s : set α), s nhds x (∃ᶠ (n : ) in filter.at_top, f^[n] x s)

Poincaré recurrence theorem. Let f : α → α be a conservative dynamical system on a topological space with second countable topology and measurable open sets. Then almost every point x : α is recurrent: it visits every neighborhood s ∈ 𝓝 x infinitely many times.

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theorem measure_theory.conservative.iterate {α : Type u_2} {f : α α} {μ : measure_theory.measure α} (hf : μ) (n : ) :

Iteration of a conservative system is a conservative system.