A diffeomorphism f has a heterodimensional cycle if there are (transitive) hyperbolic sets Λ and Σ having different indices (dimension of the unstable bundle) such that the unstable manifold of Λ meets the stable one of Σ and vice versa. This cycle has co-index 1 if index(Λ) = index(Σ) ± 1. This cycle is robust if, for every g close to f , the continuations of Λ and Σ for g have a heterodimensional cycle.We prove that any co-index 1 heterodimensional cycle associated with a pair of hyperbolic saddles generates C 1 -robust heterodimensioal cycles. Therefore, in dimension three, every heterodimensional cycle generates robust cycles.We also derive some consequences from this result for C 1 -generic dynamics (in any dimension). Two of such consequences are the following. For tame diffeomorphisms (generic diffeomorphisms with finitely many chain recurrence classes) there is the following dichotomy: either the system is hyperbolic or it has a robust heterodimensional cycle. Moreover, any chain recurrence class containing saddles having different indices has a robust cycle.
471Question 1.2. Let M be closed manifold. Does it exist a C 1 -open and dense subset O ⊂ Diff 1 (M ) such that every f ∈ O either verifies the Axiom A and the no-cycles condition or has a C 1 -robust heterodimensional cycle?Note that a positive answer to this question implies the C 1 -density of hyperbolic surface diffeomorphisms. See the discussion in § 1.3 about the Smale density conjecture. We will see that Theorem 1.14 gives a partial positive answer to this question for the so-called tame diffeomorphisms (diffeomorphisms finitely many homoclinic classes, see the preciseThe examples by Abraham-Smale of non-Axiom A diffeomorphisms involves a hyperbolic set Γ whose unstable manifold has dimension strictly greater than the dimension of its unstable bundle. Note that a normally hyperbolic extension of transitive Anosov diffeomorphisms on a torus T 2 gives an example of this configuration.The construction in [8] gives a slightly different mechanism for constructing non-Axiom A diffeomorphisms and robust heterodimensional cycles, based on the notion of blender. Roughly speaking, a blender is a hyperbolic set whose embedding in the ambient manifold verifies some specific geometric properties, whose effect is that, as in the Abraham-Smale example, the unstable manifold of a blender looks like a manifold of higher dimension. We review the construction and main properties of blenders in § 4.1.3. See also [15, Chapter 6.1] for a discussion of this notion.One of the goals of this paper is to show that blenders (and as a consequence robust heterodimensional cycles) appear in a natural way in the unfolding of heterodimensional cycles associated with two saddles.
Definition 1.3 (heterodimensional cycle and co-index 1 cycle).A diffeomorphism f has a heterodimensional cycle (see Figure 1) associated with two hyperbolic periodic saddles P and Q of f if the saddles P and Q have different indices, the stable manifold of the orbit of P meets the unstable manifold of...