This paper is concerned with
series expansions of the form f(z) =∑0∞hkpk(z), where the functions {pk} are
analytic and satisfy a certain asymptotic condition. Relationships between the space
ℱ of expandable functions, the coefficient space ℋ, and the matrix operator
Bjk= pk(j)(0) are studied, and ℱ is shown to be a Banach space isomorphic to c0,
the space of complex sequences with limit 0. Necessary and sufficient conditions for
convergence of ∑0∞hkpk(z) are given in terms of the coefficient sequence
h.