Partial AHS-Structures, their Cartan description and partial BGG sequences

Abstract

G-structures and Cartan geometries are two major approaches to the description of geometric structures (in the sense of differential geometry) on manifolds of some fixed dimension $n$. We show that both descriptions naturally extend to the setting of manifolds of dimension $\geq n$ which are endowed with a distinguished involutive distribution $F$ of rank $n$. The resulting ``partial'' structures are most naturally interpreted as smooth families of standard G-structures or Cartan geometries on the leaves of the foliation defined by $F$. We prove that for the special class of AHS-structures (also known as $|1|$-graded parabolic geometries) the construction of a canonical Cartan geometry associated to a G-structure extends to this general setting. As an application, we prove that for partial AHS-structures there is an analog of the machinery of BGG sequences. This constructs sequences of differential operators of arbitrarily high order intrinsic to the structures. Under appropriate flatness conditions, these sequence are fine resolutions of sheaves which locally can be realized as pullbacks of sheaves on local leaf spaces for the foliation defined by $F$.