In this article, we investigate when the set of primitive geodesic lengths on a Riemannian manifold have arbitrarily long arithmetic progressions. We prove that in the space of negatively curved metrics, a metric having such arithmetic progressions is quite rare. We introduce almost arithmetic progressions, a coarsification of arithmetic progressions, and prove that every negatively curved, closed Riemannian manifold has arbitrarily long almost arithmetic progressions in its primitive length spectrum. Concerning genuine arithmetic progressions, we prove that every non-compact, locally symmetric, arithmetic manifold has arbitrarily long arithmetic progressions in its primitive length spectrum. We end with a conjectural characterization of arithmeticity in terms of arithmetic progressions in the primitive length spectrum. We also suggest an approach to a well known spectral rigidity problem based on the scarcity of manifolds with arithmetic progressions.
IntroductionGiven a Riemannian manifold M, the associated geodesic length spectrum is an invariant of central importance. When the manifold M is closed and equipped with a negatively curved metric, there are several results that show primitive, closed geodesics on M play the role of primes in Z (or prime ideals in O K ). Prime geodesic theorems like Huber [19], Margulis [25], and Sarnak [36] on growth rates of closed geodesics of length at most t are strong analogs of the prime number theorem (see, for instance, also [8], [30], [39], and [40]). Sunada's construction of length isospectral manifolds [41] was inspired by a similar construction of non-isomorphic number fields with identical Dedekind ζ -functions (see [28]). The Cebotarev density theorem has also been extended in various directions to lifting behavior of closed geodesics on finite covers (see [42]). There are a myriad of additional results, and this article continues to delve deeper into this important theme. Let us start by introducing some basic terminology:Definition. Let (M, g) be a Riemannian orbifold, and [g] a conjugacy class inside the orbifold fundamental group π 1 (M). We let L [g] ⊂ [0, ∞) consist of the lengths of all closed orbifold geodesics in M which represent the conjugacy class [g]. This could be empty if M is non-compact, and if M is a compact manifold (rather than orbifold), then L [g] takes values in R + := (0, ∞). The length spectrum of (M, g) is the multiset L (M, g) obtained by taking the union of all the sets L [g] , where [g] ranges over all conjugacy classes in M.We say a conjugacy class [g] is primitive if the element g is not a proper power of some other element (in particular g must have infinite order). The primitive length spectrum of (M, g) is the multiset L p (M, g) obtained by taking the union of all the sets L [g] , where [g] ranges over all primitive conjugacy classes in M.