The rotation vector of the trihedron of a curve with curvature 
when a point moves along a curve with unit speed. It is given by
 | (1) |
where 
is the torsion, 
the tangent vector, and 
the binormal vector. The Darboux vector field satisfies
 |  |  | (2) |
 |  |  | (3) |
 |  |  | (4) |
