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)
|
