The idea is, divide as × c, so that we can project every point p of to , with c to a single point (0,0). Then define a function like on which has a single critical point at (0, 0).
The idea is, divide as × c, so that we can project every point p of to , with c to a single point (0,0). Then define a function like on which has a single critical point at (0, 0).