For the Bernstein polynomial
approximation process on a simplex or a cube, the class of functions yielding optimal
approximation will be given. That is, we will find the class of functions for which
∥Bnf − f∥C(S)= O(n−1) in terms of the behaviour of a certain K-functional.
Moreover, this is done in the context of direct and converse results which yields an
improvement on such results as well.