Unit equations and Fermat surfaces in positive characteristic

In this article we study the three-variable unit equation $x+y+z=1$ to be solved in $x,y,z\in {\mathcal{O}}_S^{\ast }$, where ${\mathcal{O}}_S^{\ast }$ is the $S$-unit group of some global function field. We give upper bounds for the height of solutions and the number of solutions. We also apply these techniques to study the Fermat surface $x^N+y^N+z^N=1$.