Let (X ) be a metric space and T : X → X a continuous map. If the sequence (T ) ∈N of iterates of T is pointwise convergent in X , then for any ∈ X , the limitis a fixed point of T . The problem of determining the form of µ T leads to the invariance equation µ T • T = µ T , which is difficult to solve in general if the set of fixed points of T is not a singleton. We consider this problem assuming that X = I , where I is a real interval, ≥ 2 a fixed positive integer and T is the mean-type mapping M =(M 1 M ) of I . In this paper we give the explicit forms of µ M for some classes of mean-type mappings. In particular, the classical Pythagorean harmony proportion can be interpreted as an important invariance equality. Some applications are presented. We show that, in general, the mean-type mappings are not non-expansive.
MSC:26E60, 26A18