We construct symbolic dynamics on sets of full measure (with respect to an ergodic measure of positive entropy) for C 1+ε flows on closed smooth three dimensional manifolds. One consequence is that the geodesic flow on the unit tangent bundle of a closed C ∞ surface has at least const ×(e hT /T ) simple closed orbits of period less than T , whenever the topological entropy h is positive -and without further assumptions on the curvature. Kat82]. It extends to flows of lesser regularity, under the additional assumption that they possess a measure of maximal entropy (Theorem 8.1). The lower bound Ce ht /T is sharp in many special cases [Hub59, Mar69, PP83, Kni97], but not in the general setup of this paper. For more on this, see §8.
The theorem strengthens Katok's bound lim infWe obtain Theorem 1.1 by constructing a symbolic model that is a finite-to-one extension of ϕ. The orbits of this model are easier to understand than those of the original flow. This technique, called "symbolic dynamics", can be traced back to the work of Hadamard, Morse, Artin, and Hedlund.We proceed to describe the symbolic models used in this work. Let G be a directed graph with a countable set of vertices V . We write v → w if there is an edge from v to w, and we assume throughout that for every v there are u, w s.t.Birkhoff cocycle: Suppose r : Σ → R is a function. The Birkhoff sums of r are r n := r + r • σ + · · · + r • σ n−1 (n ≥ 1). There is a unique way to extend the definition to n ≤ 0 in such a way that the cocycle identity r m+n = r n + r m • σ n holds for all m, n ∈ Z: r 0 := 0 and r n := −r |n| • σ −|n| (n < 0). Topological Markov flow: Suppose r : Σ → R + is Hölder continuous and bounded away from zero and infinity. The topological Markov flow with roof function r and base map σ : Σ → Σ is the flow σUniform Poincaré section: The Poincaré section Λ is called uniform if its roof function is bounded away from zero and infinity. If Λ is uniform, then µ Λ is finite and it can be normalized. With this normalization, for every Borel subset E ⊂ Λ and 0All the Poincaré sections considered in this paper will be uniform, and each of them will be the disjoint union of finitely many embedded smooth two dimensional discs. Let ∂Λ denote the union of the boundaries of these discs. The set ∂Λ will introduce discontinuities to the Poincaré map of Λ. Singular set: The singular set of a Poincaré section Λ is S(Λ) := p ∈ Λ : p does not have a relative neighborhood V ⊂ Λ \ ∂Λ s.t. V is diffeomorphic to an open disc, and f Λ : V → f Λ (V ) and f −1 Λ : V → f −1 Λ (V ) are diffeomorphisms . Regular set: Λ := Λ \ S(Λ). SYMBOLIC DYNAMICS FOR FLOWS 5 Basic constructions. Let ϕ be a flow satisfying our standing assumptions.Canonical transverse disc:The following lemmas are standard, see the appendix for proofs.Lemma 2.1. There is a constant r s > 0 which only depends on M and ϕ s.t. for every p ∈ M and 0 < r < r s , S := S r (p) is a C ∞ embedded closed disc, | (X q , T q S)| ≥ 1 2 radians for all q ∈ S, and dist M (·, ·) ≤ dist S (·, ·) ≤ 2 d...