  A class of continuous controllers termed Robust Integral of the Signum of the
Error (RISE) have been published over the last decade as a means to yield
asymptotic convergence of the tracking error for classes of nonlinear systems
that are subject to exogenous disturbances and/or modeling uncertainties. The
development of this class of controllers relies on a property related to the
integral of the signum of an error signal. A proof for this property is not
available in previous literature. The stability of some RISE controllers is
analyzed using differential inclusions. Such results rely on the hypothesis
that a set of points is Lebesgue negligible. This paper states and proves two
lemmas related to the properties.
