We study several properties of the normal Lebesgue trace of vector fields introduced
by the second and third author, De Rosa and Inversi (2024), in the context of the
energy conservation for the Euler equations in Onsager-critical classes. Among other
things, we prove that the normal Lebesgue trace satisfies the Gauss–Green identity
and, by providing explicit counterexamples, that it is a notion sitting strictly between
the distributional one for measure-divergence vector fields and the strong one
for
functions. These results are then applied to the study of the uniqueness
of weak solutions for continuity equations on bounded domains, allowing
for the removal of the assumption in Crippa et el. (2014a) of global
regularity up
to the boundary, at least around the portion of the boundary where the characteristics exit
the domain or are tangent. The proof relies on an explicit renormalization formula completely
characterized by the boundary datum and the positive part of the normal Lebesgue
trace. In the case when the characteristics enter the domain, a counterexample shows that
achieving the normal trace in the Lebesgue sense is not enough to prevent nonuniqueness,
and thus a
assumption seems to be necessary to get uniqueness.
Keywords
normal traces, continuity equations, uniqueness vs.
nonuniqueness, $BV$ vector fields