Dr. Bisson recommended looking at the following:

It mentions an interesting problem (DGP). Given six points,

*A*,*B,**C*, D,*E,**F*, we can ask some questions:- are there six edge lengths for a possible degenerate tetrahedron?
- does the tetrahedron have zero volume and thus lie in a plane?
- if so, which corner lies inside the triangle of the other three corners?

There are degenerate versions of the last of these questions, since three corners may lie in a line.

We think that there are good answers for these questions, and they should help us understand collisions in stick robots and stick knots.