ABSTRACT. In the first half of the paper we construct a Morse-type theory on certain spaces of braid diagrams. We define a topological invariant of closed positive braids which is correlated with the existence of invariant sets of parabolic flows defined on discretized braid spaces. Parabolic flows, a type of one-dimensional lattice dynamics, evolve singular braid diagrams in such a way as to decrease their topological complexity; algebraic lengths decrease monotonically. This topological invariant is derived from a MorseConley homotopy index.In the second half of the paper we apply this technology to second order Lagrangians via a discrete formulation of the variational problem. This culminates in a very general forcing theorem for the existence of infinitely many braid classes of closed orbits.
PRELUDEIt is well-known that under the evolution of any scalar uniformly parabolic equation of the form (1) u t = f (x, u, u x , u xx ) ; ∂ uxx f ≥ δ > 0, the graphs of two solutions u 1 (x, t) and u 2 (x, t) evolve in such a way that the number of intersections of the graphs does not increase in time. This principle, known in various circles as "comparison principle" or "lap number" techniques, entwines the geometry of the graphs (u xx is a curvature term), the topology of the solutions (the intersection number is a local linking number), and the local dynamics of the PDE. This is a valuable approach for understanding local dynamics for a wide variety of flows exhibiting parabolic behavior with both classical [54] and contemporary [41,5,10,19] implications. This paper is an extension of this local technique to a global technique. One such well-established globalization appears in the work of Angenent on curve-shortening [4]: evolving closed curves on a surface by curve shortening isolates the classes of curves dynamically and implies a monotonicity with respect to number of self-intersections.In contrast, one could consider the following topological globalization. Superimposing the graphs of a collection of functions u α (x) gives something which resembles the projection of a topological braid onto the plane. Assume that the "height" of the strands above the page is given by the slope u α x (x), or, equivalently, that all of the crossings in the projection are of the same sign (bottom-over-top): see Fig. 1[left]. Evolving these functions under a parabolic equation (with, say, boundary endpoints fixed) yields a flow on a certain space of braid diagrams which has a topological monotonicity: linking can be destroyed but not created. This establishes a partial ordering on the semigroup of positive braids which is respected by parabolic dynamics. The idea of topological braid classes with this partial ordering is a globalization of the lap number (which, in braid-theoretic terms becomes the length of the braid in the braid group under standard generators).1.1. Parabolic flows on spaces of braid diagrams. In this paper, we initiate the study of parabolic flows on spaces of braid diagrams. The particular braids in question wi...