Abstract. For a non-empty compact set A ⊂ R d , d ≥ 2, and r ≥ 0, let A ⊕r denote the set of points whose distance from A is r at the most. It is well-known that the volume, V d (A ⊕r ), of A ⊕r is a polynomial of degree d in the parameter r if A is convex. We pursue the reverse question and ask whether A is necessarily convex if V d (A ⊕r ) is a polynomial in r. An affirmative answer is given in dimension d = 2, counterexamples are provided for d ≥ 3. A positive resolution of the question in all dimensions is obtained if the assumption of a polynomial parallel volume is strengthened to the validity of a (polynomial) local Steiner formula. (2000): 52A38, 28A75, 52A22, 53C65
Mathematics Subject Classification