Alain Connes introduced the use of Lie groupoids in noncommutative geometry in his pioneering work on the index theory of foliations. In the present paper, we recall the basic notion involved: groupoids, their C * -algebras, their pseudodifferential calculus... We review several recent and older advances on the involvement of Lie groupoids in noncommutative geometry. We then propose some open questions and possible developments of the subject.
Lie groupoids and their operators algebrasWe refer to [78,80] for the classical definitions and constructions related to groupoids and their Lie algebroids. The construction of the C * -algebra of a groupoid is due to Jean Renault [100], one can look at his course [101] which is mainly devoted to locally compact groupoids.
Lie groupoids
GeneralitiesA groupoid is a small category in which every morphism is an isomorphism. Thus a groupoid G is a pair (G (0) , G (1) ) of sets together with structural morphisms:Units and arrows. The set G (0) denotes the set of objects (or units) of the groupoid, whereas the set G (1) is the set of morphisms (or arrows). The unit map u : G (0) → G (1) is the injective map which assigns to any object of G its identity morphism.The source and range maps s, r : G (1) → G (0) are (surjective) maps equal to identity in restriction toThe inverse ι : G (1) → G (1) is an involutive map which exchanges the source and range:for α ∈ G, (α −1 ) −1 = α and s(α −1 ) = r(α), where α −1 denotes ι(α)