Topological Drawings meet Classical Theorems of Convex Geometry - PowerPoint PPT Presentation

Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold 1 , Stefan Felsner 2 , Manfred Scheucher 2 , oder 2 , Raphael Steiner 2 Felix Schr¨ 1 FernUniversit¨ at in Hagen 2 Technische Universit¨ at Berlin

Theorems of Convex Geometry • Carath´ eodory’s Theorem • Colorful Carath´ eodory Theorem • Helly’s Theorem • Radon’s Theorem • Tverberg’s Theorem MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Theorem (Carath´ eodory) For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Theorem (Carath´ eodory) For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . p 0 p 1 p 2 MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Theorem (Carath´ eodory) For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . p 0 p 1 p 2 MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Theorem (Carath´ eodory) For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . p 0 p 1 p 2 MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Theorem (Carath´ eodory) For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . p 0 p 1 p 2 MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Topological Drawing A topological drawing is a drawing of a complete graph such that: MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Hierarchy [Arroyo, McQuillan, Richter, Salazar ’17] 1. Topological drawing 2. Convex drawing: every triangle has a convex side . y y x x MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Hierarchy [Arroyo, McQuillan, Richter, Salazar ’17] 1. Topological drawing 2. Convex drawing: every triangle has a convex side . 3. Pseudocircular drawing: every edge can be extended to a pseudocircle. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Hierarchy [Arroyo, McQuillan, Richter, Salazar ’17] 1. Topological drawing 2. Convex drawing: every triangle has a convex side . 3. Pseudocircular drawing: every edge can be extended to a pseudocircle. 4. Pseudolinear drawing: every edge can be extended to a pseudoline. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Hierarchy [Arroyo, McQuillan, Richter, Salazar ’17] 1. Topological drawing 2. Convex drawing: every triangle has a convex side . 3. Pseudocircular drawing: every edge can be extended to a pseudocircle. 4. Pseudolinear drawing: every edge can be extended to a pseudoline. 5. straightline drawing MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem Theorem For P ⊆ R 2 and a point x ∈ conv P there are points p 0 , p 1 , p 2 ∈ P such that x is inside the triangle p 0 , p 1 , p 2 . p 0 p 1 p 2 MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem in Topological Drawings Theorem (Balko, Fulek, Kynˇ cl ’15) Let D be a topological drawing of K n , x ∈ R 2 a point in a bounded connected component of R 2 − D. Then there is a triangle which contains x in the interior. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem in Topological Drawings Theorem (Balko, Fulek, Kynˇ cl ’15) Let D be a topological drawing of K n , x ∈ R 2 a point in a bounded connected component of R 2 − D. Then there is a triangle which contains x in the interior. Our Proof. • ( D , x ) minimal choice violating the claim MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem in Topological Drawings Theorem (Balko, Fulek, Kynˇ cl ’15) Let D be a topological drawing of K n , x ∈ R 2 a point in a bounded connected component of R 2 − D. Then there is a triangle which contains x in the interior. Our Proof. • ( D , x ) minimal choice violating the claim • Delete edges from one vertex a such that x is still in a bounded cell. Call this drawing D ′ . If we delete another edge e = ab , x is in the outer cell. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem in Topological Drawings Theorem (Balko, Fulek, Kynˇ cl ’15) Let D be a topological drawing of K n , x ∈ R 2 a point in a bounded connected component of R 2 − D. Then there is a triangle which contains x in the interior. Our Proof. • ( D , x ) minimal choice violating the claim • Delete edges from one vertex a such that x is still in a bounded cell. Call this drawing D ′ . If we delete another edge e = ab , x is in the outer cell. • Then there is a path P in D ′ − ab which connects x to infinity. Choose P with no crossings in D ′ − ab and minimal crossings with ab . MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Carath´ eodory’s Theorem in Topological Drawings Theorem (Balko, Fulek, Kynˇ cl ’15) Let D be a topological drawing of K n , x ∈ R 2 a point in a bounded connected component of R 2 − D. Then there is a triangle which contains x in the interior. Our Proof. • ( D , x ) minimal choice violating the claim • Delete edges from one vertex a such that x is still in a bounded cell. Call this drawing D ′ . If we delete another edge e = ab , x is in the outer cell. • Then there is a path P in D ′ − ab which connects x to infinity. Choose P with no crossings in D ′ − ab and minimal crossings with ab . • Claim: P has exactly one crossing with ab . MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

b P x a • a is the vertex, we started to delete edges, i.e. b is still connected to all vertices in D ′ • P does not cross an edge in D ′ − ab and minimal number of crossings in D ′ with the edge ab • Consider another path P ′ with fewer crossings with the edge ab . By Minimality an edge crossing P ′ exists • c is connected to b . Contradiction. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

b P x a • a is the vertex, we started to delete edges, i.e. b is still connected to all vertices in D ′ • P does not cross an edge in D ′ − ab and minimal number of crossings in D ′ with the edge ab • Consider another path P ′ with fewer crossings with the edge ab . By Minimality an edge crossing P ′ exists • c is connected to b . Contradiction. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

b c P x a • a is the vertex, we started to delete edges, i.e. b is still connected to all vertices in D ′ • P does not cross an edge in D ′ − ab and minimal number of crossings in D ′ with the edge ab • Consider another path P ′ with fewer crossings with the edge ab . By Minimality an edge crossing P ′ exists • c is connected to b . Contradiction. MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

1. If a has another neighbor c in D ′ , b P x c a MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

1. If a has another neighbor c in D ′ , b P x c a MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

1. If a has another neighbor c in D ′ , we get a triangle such that x is in the interior. 2. If b is the only neighbor of a in D ′ : b p P x c d a MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

1. If a has another neighbor c in D ′ , we get a triangle such that x is in the interior. 2. If b is the only neighbor of a in D ′ : b P x c d a MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

1. If a has another neighbor c in D ′ , we get a triangle such that x is in the interior. 2. If b is the only neighbor of a in D ′ : b P x c d a MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Colorful Carath´ eodory Theorem (B´ ar´ any ’82) Consider the sets P 0 , P 1 , P 2 ⊂ R 2 , x ∈ R 2 . If x ∈ conv P i for all i ∈ { 0 , 1 , 2 } , there are p i ∈ P i for every i ∈ { 0 , 1 , 2 } such that x ∈ conv { p 0 , p 1 , p 2 } . MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold

Colorful Carath´ eodory Theorem (B´ ar´ any ’82) Consider the sets P 0 , P 1 , P 2 ⊂ R 2 , x ∈ R 2 . If x ∈ conv P i for all i ∈ { 0 , 1 , 2 } , there are p i ∈ P i for every i ∈ { 0 , 1 , 2 } such that x ∈ conv { p 0 , p 1 , p 2 } . MCW 2019 Topological Drawings meet Classical Theorems of Convex Geometry Helena Bergold