Analogies of Curvature and Torsion for Polygonal Lines

Let POPI . . .P, be a polygonal line formed by segments a,+] = PiP;,-l, i = 0,. . . , n - 1. Instead of the curvature and torsion of regular curves we will define two sequences of angles: p;, i = 1, . . . , n - 1 and $,, j = l , . . . , n - 2.