Fair partitioning by straight lines

TL;DR

This paper proves that any convex pizza can be fairly partitioned into an even number of slices with straight lines, using a specific cutting and moving process, and establishes the necessary condition that the number of slices must be even.

Contribution

It demonstrates the existence of fair straight-line partitions for convex pizzas into an even number of slices and introduces a key geometric result about $eta$-sections of convex bodies.

Findings

Fair partition exists if and only if the number of slices is even.

A key geometric lemma about $eta$-sections of convex bodies is proven.

The method applies to convex bodies in the plane, with open questions for non-planar cases.

Abstract

A pizza is a pair of planar convex bodies $A\subseteq B$,where $B$ represents the dough and $A$ the topping of the pizza. A partition of a pizza by straight lines is a succession of double operations:a cut by a full straight line, followed by a Euclidean move of one of theresulting pieces; then the procedure is repeated.The final partition is said to be fair if each resulting slice has the same amount of $A$ and the same amount of $B$.This note proves that, given an integer $n\geq2$, there exists a fair partition by straight lines of any pizza $(A,B)$ into $n$ parts if and onlyif $n$ is even.The proof uses the following result:For any planar convex bodies $A, B$ with $A\subseteq B$, and any$\alpha\in\,]0,\frac12[\,$, there exists an $\alpha$-section of $A$ which is a$\beta$-section of $B$ for some $\beta\geq\alpha$. (An $\alpha$-section of $A$ is a straight line cutting $A$ into two parts, one of which has area $\alpha|A|$.)The question remains open if the word "planar" is dropped.