  This paper proposes a unifying framework for the convergence analysis of a
class of adaptive optimal identifiers. The considered class of identifiers is
constructed from the sequence of minimizing sets of a family of objective
functions. For the purpose of the analysis we introduce a generalized version
of the classical persistence of excitation condition. Based on this an
Input-to-State-Stability (ISS)-type of property is derived for the studied
class of adaptive estimators.
