This paper deals with spectral
properties of commutative locally holomorphic Banach algebra valued functions. One
of the main concepts is that of a spectral set of such a function. This concept, which
is due to L. Mittenthal, extends that of a spectral set of a single Banach algebra
element. It will be shown that the spectral idempotent associated with a nonvoid
spectral set is nonzero. This result is a generalization of a well-known theorem in
ordinary spectral theory. It will be used to prove a correctly stated but incorrectly
proven theorem of L. Mittenthal.