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).
Wang and Zelenko (2026) constructed an equiregular rank-two example with growth vector 
on a manifold diffeomorphic to 
for which the sub-Riemannian
Sard conjecture fails.
