  We revisit the classical issue of the controllability/observability cost of
linear first order evolution systems, starting with ODEs, before turning to
some linear first order evolution PDEs in several space dimensions, including
hyperbolic systems and pseudo-differential systems obtained by linearization in
fluid mechanics. In particular we investigate the cost of localized initial
data, and in the dispersive case, of initial data which are semi-classically
microlocalized.
