The topology of configuration spaces and their free and based loop spaces plays an important role in the study of the existence of periodic solutions of Hamiltonian systems of n-body type (see e.g. [1], [6], [7]). In particular, if Fn(R) is the configuration space of n-particles u = (u1, . . . , un) in R, then the free loop space ΛFn(R) is, up to homotopy type, the domain of a functional f whose critical points are solutions of a corresponding n-body problem. Thus, the homotopy-type invariants of ΛFn(R) such as Lusternik–Schnirelmann (LS) category (and its generalizations — commonly called “index theories”), homotopy and homology play an important role in the subject. In some recent work ([15], [16], [17]), P. Majer and S. Terracini introduced an interesting “collision index” in the space ΛFn(R) which allowed contractions of subsets of ΛFn(R) to move through subspaces intermediate to ΛFn(R) and Λ(R), thus allowing a limited number of collisions during the contraction. However, their index is based upon an equivalence relation which results in a generalized notion of collision. Namely, that ui “collides” with uj during the deformation h if there is a chain of indices i1, . . . , is, such that ui = ui1 , uj = uis and uir collides with uir+1 , 1 ≤ r ≤ s.
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