How to calculate the volume of a special polyhedron

   ----A question on Researchgate Net

     In my paper arXiv1705.08064(On Controllable Abundance Of Saturated-input Linear Discrete Systems), a special parallel polyhedron in n-dimesions space is difinted as C={c1a1+c2a2+...+cnan, for all ci:0=<ci<=1}, where ai(i=1,2,...,m) are the n-dimisions vector and m>n. Because the control ability of the dynamic systems in the control field is related to the volume of the special polyhedron, the volume computing is a key problem in the analysis and design of the control systems. How to calculate fastly and effectively the volume of the special polyhedron?