Geometry of the minimal solutions of a linear Diophantine Equation

Let $a_1,\ldots,a_n$ and $b_1,\ldots,b_m$ be fixed positive integers, and let ${\mathcal S}$ denote the set of all nonnegative integer solutions of the equation $x_1a_1+\ldots +x_na_n=y_1b_1+\ldots +y_mb_m$. A solution $(x_1,\ldots,x_n,y_1,\ldots,y_m)$ in ${\mathcal S}$ is called $\textit{minimal}$ if it cannot be expressed as the sum of two nonzero solutions in ${\mathcal S}$. For each pair $(i,j)$ with $1\leq i\leq n$ and $1\leq j\leq m$, the solution whose only nonzero coordinates are $x_i=b_j$ and $y_j=a_i$ is called a $\textit{generator}$. Our main result shows that every minimal solution is a convex combination of the generators and the zero-solution. This proves a conjecture of Henk-Weismantel and, independently, Ho\c{s}ten-Sturmfels.