Lecture17

# Lecture17 - C&O 355 Lecture 17 Notes Nicholas Harvey...

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Unformatted text preview: C&O 355: Lecture 17 Notes Nicholas Harvey http://www.math.uwaterloo.ca/~harvey/ 1 Faces of Polyhedra Definition 1.1. An affine space A ⊆ R n is any set of the form A = { x + z : x ∈ L } , where L ⊆ R n is a linear subspace and z ∈ R n is any vector. The dimension of A , denoted dim A , is simply the dimension of the corresponding linear subspace L . Definition 1.2. Let C ⊆ R n be any set. The dimension of C , denoted dim C , is min { dim A : C ⊆ A } , where the minimum is taken over all affine spaces A . Strictly speaking, if C = ∅ then dim C is not defined. We will adopt the convention that dim C =- 1. Definition 1.3. Let C ⊆ R n be any convex set. An inequality a T x ≤ b is called a valid inequality if a T x ≤ b holds for every point x ∈ C . Definition 1.4. Let P ⊆ R n be a polyhedron. A face of P is any set F of the form F = P ∩ n x ∈ R n : a T x = b o , (1.1) where a T x ≤ b is a valid inequality for P . Notice that any face of a polyhedron is itself a polyhedron. As an example, if we take a = and b = 0 then this shows that P is itself a face. On the other hand, if we take a = and b = 1 then this shows that ∅ is a face. Definition 1.5. Let P be a polyhedron and let F ⊆ P be a face. Then F is called a d- dimensional face or a d-face if dim F = d . Note that if F consists of a single point v (i.e., F = { v } ) then F is a 0-face. In this case, v is the unique maximizer of a over P , where a is the vector in Eq. (1.1), and thus v is a vertex of P . Recalling our earlier results, we see that for polyhedra, vertices, extreme points, basic feasible solutions, and 1-faces are the same concept. Definition 1.6. If dim P = d and F ⊂ P is a ( d- 1)-face then F is called a facet . Definition 1.7. A 1-face of a polyhedron is called an edge . 1 Fact 1.8. Let P be a polytope with dim P = d and let F be a face of P . Then F is itself a polytope and so it too has faces. The faces of F are precisely the faces of P that are contained in F . Now assume that F is a facet. The facets of F are precisely the ( d- 2)-faces of P that are contained in F . Furthermore, each facet of F can be obtained as the intersection of F and another facet of P ....
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Lecture17 - C&O 355 Lecture 17 Notes Nicholas Harvey...

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