# Helly's theorem

This is the last of a small series of similar and basic results in convex geometry.

Helly's theorem: Let \(X_1,\ldots,X_n\) be a finite collection of convex subsets of \(R^d\). If the intersection of every \(d+1\) of these sets is nonempty, then the whole collection has a nonempty intersection.

Proof:

Below we show that if every \(k\geq d+1\) sets have nonempty intersection, then so do every \(k+1\) sets.

It suffices to consider only \(k+1\geq d+2\) convex sets \(X_1,\ldots,X_{k+1}\).

Let \(C_i=\bigcap_{v\neq i}X_v\)

Clearly if any two distinct \(C_i\) and \(C_j\) overlap then we are done, so assume they don't and pick a representative point \(p_i\) for \(C_i\) for each \(i\). Now apply Radon's theorem to \(p_1,\ldots,p_{k+1}\), i.e. without loss of generality there is a point \(x\) that is convex combination of \(p_1,\ldots,p_m\), and also convex combination of \(p_{m+1},\ldots,p_{k+1}\). For any \(i\in[1,m]\), the latter ensures \(x\in X_i\), and for any \(i\in[m+1,k+1]\), the former implies the same thing.

Q.E.D.

Helly's theorem: Let \(X_1,\ldots,X_n\) be a finite collection of convex subsets of \(R^d\). If the intersection of every \(d+1\) of these sets is nonempty, then the whole collection has a nonempty intersection.

Proof:

Below we show that if every \(k\geq d+1\) sets have nonempty intersection, then so do every \(k+1\) sets.

It suffices to consider only \(k+1\geq d+2\) convex sets \(X_1,\ldots,X_{k+1}\).

Let \(C_i=\bigcap_{v\neq i}X_v\)

Clearly if any two distinct \(C_i\) and \(C_j\) overlap then we are done, so assume they don't and pick a representative point \(p_i\) for \(C_i\) for each \(i\). Now apply Radon's theorem to \(p_1,\ldots,p_{k+1}\), i.e. without loss of generality there is a point \(x\) that is convex combination of \(p_1,\ldots,p_m\), and also convex combination of \(p_{m+1},\ldots,p_{k+1}\). For any \(i\in[1,m]\), the latter ensures \(x\in X_i\), and for any \(i\in[m+1,k+1]\), the former implies the same thing.

Q.E.D.

post by Shen-Fu Tsai