Intersection points of planar curves can be computed

Abstract

Consider two paths $\phi,\psi:[0;1]\to [0;1]^2$ in the unit square such that $\phi(0)=(0,0)$, $\phi(1)=(1,1)$, $\psi(0)=(0,1)$ and $\psi(1)=(1,0)$. By continuity of $\phi$ and $\psi$ there is a point of intersection. We prove that from $\phi$ and $\psi$ we can compute closed intervals $S_\phi,S_\psi \subseteq [0;1]$ such that $\phi(S_\phi)=\psi(S_\psi)$.