An equiregular distribution is a smooth distribution for which the dimension of each successive span of iterated Lie
brackets is constant over the manifold. For a bracket-generating distribution , write
for the span of brackets of length at most
at
.
Equiregularity means that the growth vector
is independent of
(Wang and Zelenko 2026).
The sub-Riemannian Sard conjecture concerns endpoints of abnormal curves for such distributions.