  This article studies hypoellipticity on general filtered manifolds. We extend
the Rockland criterion to a pseudodifferential calculus on filtered manifolds,
construct a parametrix and describe its precise analytic structure. We use this
result to study Rockland sequences, a notion generalizing elliptic sequences to
filtered manifolds. The main application that we present is to the analysis of
the Bernstein--Gelfand--Gelfand (BGG) sequences over regular parabolic
geometries. We do this by generalizing the BGG machinery to more general
filtered manifolds (in a non-canonical way) and show that the generalized BGG
sequences are Rockland in a graded sense.
