Abstract. For a convex domain D that is enclosed by the hypersurface ∂D of bounded normal curvature, we prove an angle comparison theorem for angles between ∂D and geodesic rays starting from some fixed point in D, and the corresponding angles for hypersurfaces of constant normal curvature. Also, we obtain a comparison theorem for support functions of such surfaces. As a corollary, we present a proof of Blaschke's Rolling Theorem.
Preliminaries and the main resultsIs it known the following theorem due to W. Blaschke:be a convex body with the C r -smooth boundary ∂D (r 2), and P ∈ ∂D be an arbitrary point. Let ∂D λ ⊂ M m (c) be a complete hypersurface of constant normal curvature equal to some λ > 0, and suppose that ∂D λ touches ∂D at P so that their inner unit normals coincide.A. If normal curvatures k n of the hypersurface ∂D at all points and in all directions satisfy the inequality k n λ, then ∂D lies entirely in the closed convex domain bounded by ∂D λ .B. If normal curvatures of the hypersurface ∂D at all points and in all directions satisfy the inequality λ k n , then the hypersurface ∂D λ lies in D.Moreover, the hypersurfaces ∂D and ∂D λ can intersect only by a domain that contains the point P .For the Euclidean space this theorem was first proved in [1]; for the general case of constant curvature spaces see [2,3,4].It appears that Blaschke's Rolling Theorem can be obtained as a corollary from the following comparison theorems for angles between the radius-vector of a hypersurface and its normals. In order to give exact statements, we need to agree on some notations.Everywhere below let M m be a complete simply-connected m-dimensional Riemannian manifold such that its sectional curvatures K σ in a direction of a 2-plane σ ⊂ T M m satisfy the inequality c 2 K σ c 1 with some constants c 1 and c 2 .2010 Mathematics Subject Classification. 53C20.