A Bernstein-Markov Theorem for Normed Spaces

Let X and Y be real normed linear spaces and let φ : X → R be a non-negative function satisfying φ(x+ y) ≤ φ(x) + ‖y‖ for all x, y ∈ X. We show that there exist optimal constants cm,k such that if P : X → Y is any polynomial satisfying ‖P (x)‖ ≤ φ(x)m for all x ∈ X, then ‖D̂kP (x)‖ ≤ cm,kφ(x) whenever x ∈ X and 0 ≤ k ≤ m. We obtain estimates for these constants and present applications to polynomials and multilinear mappings in normed spaces.