## A course on convex geometry by Weil W.

By Weil W.

Similar geometry and topology books

Low-dimensional geometry: From Euclidean surfaces to hyperbolic knots

The examine of third-dimensional areas brings jointly components from a number of components of arithmetic. the main striking are topology and geometry, yet components of quantity concept and research additionally make appearances. some time past 30 years, there were extraordinary advancements within the arithmetic of third-dimensional manifolds.

Extra resources for A course on convex geometry

Example text

6, the latter is equivalent to the differentiability of hK at u. If all the support sets K(u), u ∈ S n−1 , of a nonempty, compact convex set K consist of points, the boundary bd K does not contain any segments. Such sets K are called strictly convex. Hence, K is strictly convex, if and only if hK is differentiable on Rn \ {0}. We finally consider the support functions of polytopes. We call a function h on Rn piecewise linear, if there are finitely many convex cones A1 , . . , Am ⊂ Rn , such that Rn = m i=1 Ai and h is linear on Ai , i = 1, .

4, there is a representation x = α1 x1 + · · · + αk xk with k ∈ N, xi ∈ A, αi > 0, and k ≥ 2, αi = 1. In case k = 1, we have x = x1 ∈ A. If x = α1 x1 + (1 − α1 ) and α2 x2 + · · · + αk xk α2 + · · · + αk α2 x2 + · · · + αk xk ∈ K. α2 + · · · + αk Since x is extreme, we obtain x = x1 ∈ A. Thus, in both cases we have x ∈ A, therefore ext K ⊂ A. 34 CHAPTER 1. CONVEX SETS In the other direction, we need only show that K = conv ext K. We prove this by induction on n. For n = 1, a compact convex subset of R1 is a segment [a, b] and ext [a, b] = {a, b}.

F) Let A be closed. e. e. A = −A). (g) Let A be closed. Then dA is a norm on Rn , if and only if A is symmetric, compact and contains 0 in its interior. (h) If A is closed, then dA is closed. 2. 2 47 Regularity of convex functions We start with a continuity property of convex functions. 1. A convex function f : Rn → (−∞, ∞] is continuous in int dom f . Proof. Let x ∈ int dom f . There exists a n-simplex P with P ⊂ int dom f and x ∈ int P . If x0 , . . , xn are the vertices of P and y ∈ P , we have y = α0 x0 + · · · + αn xn , with αi ∈ [0, 1], αi = 1, and hence f (y) ≤ α0 f (x0 ) + · · · + αn f (xn ) ≤ max f (xi ) =: c.