  The paper [TF19] proposes a data-driven control technique for single-input
single-output feedback linearizable systems with unknown control gain by
relying on a persistency of excitation assumption. This note extends those
results by showing that persistency of excitation is not necessary. We refer
the readers to the papers [TMGA17, TF19] for more background and motivation for
the technical results in this note. Conceptually, the results in this note were
greatly inspired by the work of Fliess and Join on intelligent PID controllers,
e.g., [FJ09]. Technically, we were inspired by the work of Nesic and co-workers
on observer and controller design based on approximate models [AN04, NT04] and
by the work of Astolfi and Ortega on Immersion and Invariance [AO03].
