On an arbitrary reflexive Banach space, we build asymptotic observers for an
abstract class of nonlinear control systems with possible compact outputs. An
important part of this paper is devoted to various examples, where we discuss
the existence of persistent inputs which make the system observable. These
results make a wide generalization to a nonlinear framework of previous works
on the observation problem in infinite dimension (see
[11,18,22,26,27,38,40] and other references therein).