Angles between curves in metric measure spaces
  The goal of the paper is to study the angle between two curves in the
framework of metric (and metric measure) spaces. More precisely, we give a new
notion of angle between two curves in a metric space. Such a notion has a
natural interplay with optimal transportation and is particularly well suited
for metric measure spaces satisfying the curvature-dimension condition. Indeed
one of the main results is the validity of the cosine formula on $RCD^{*}(K,N)$
metric measure spaces. As a consequence, the new introduced notions are
compatible with the corresponding classical ones for Riemannian manifolds,
Ricci limit spaces and Alexandrov spaces.
