CMPS 31306130 Computational Geometry Spring 2017 Ham Sandwich

CMPS 3130/6130 Computational Geometry Spring 2017 Ham Sandwich Theorem Carola Wenk 4/4/17 CMPS 3130/6130 Computational Geometry 1

Ham-Sandwich Theorem: Let P and Q be two finite point sets in the plane. Then there exists a line l such that on each side of l there at most |P|/2 points of P and at most |Q|/2 points of Q. 4/4/17 CMPS 3130/6130 Computational Geometry 2

Ham-Sandwich Theorem Proof: Find a line l such that on each side of l there at most |P|/2 points of P. Then rotate l counter-clockwise in such a way that there always at most |P|/2 points of P on each side of l. 4/4/17 CMPS 3130/6130 Computational Geometry 3

Rotation Left: 4 Right: 4 4/4/17 CMPS 3130/6130 Computational Geometry 4

Rotation Left: 4 Right: 3 Rotate around this point now 4/4/17 CMPS 3130/6130 Computational Geometry 5

Rotation Left: 4 Right: 4 4/4/17 CMPS 3130/6130 Computational Geometry 6

Rotation Left: 3 Right: 4 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 7

Rotation Left: 3 Right: 4 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 8

Rotation Left: 4 Right: 3 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 9

Rotation Left: 3 Right: 4 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 10

Rotation rotate 4/4/17 CMPS 3130/6130 Computational Geometry Left: 4 Right: 3 11

Rotation Left: 2 Right: 4 Rotate around this point now 4/4/17 CMPS 3130/6130 Computational Geometry 12

Rotation Left: 4 Right: 4 Rotate around this point now 4/4/17 CMPS 3130/6130 Computational Geometry 13

Rotation rotate 4/4/17 CMPS 3130/6130 Computational Geometry Left: 4 Right: 3 14

Rotation Left: 3 Right: 4 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 15

Rotation Left: 4 Right: 3 rotate 4/4/17 CMPS 3130/6130 Computational Geometry 16

Rotation Left: 4 Right: 4 4/4/17 CMPS 3130/6130 Computational Geometry 17

Proof Continued In general, choose the rotation point such that the number of points on each side of l does not change. k points m points rotate m points 4/4/17 rotate k points CMPS 3130/6130 Computational Geometry 18

Proof Continued Throughout the rotation, there at most |P|/2 points on each side of l. After 180 rotation, we get the line which is l but directed in the other direction. Let t be the number of blue points to the left of l at the beginning. In the end, t points are on the right side of l, so |Q| -t-1 are on the left side. Therefore, there must have been one orientation of l such that there were t most |Q|/2 points on each side of l. 4/4/17 CMPS 3130/6130 Computational Geometry 19