The object of this note is the
following theorem: Suppose a is a continuous affine map from a closed split face F of
a compact convex set K with values in a Banach space B enjoying the approximation
property. Suppose also that p is a strictly positive lower semi-continuous
concave function on K such that ∥a(k)∥≦ p(k) for all k in F. Then a admits a
continuous affine extension ã to K into B such that ∥ã(k)∥≦ p(k) for all k in
K.