where is the unit normal vector and is one of the two principal curvatures.
Rodrigues' Curvature Formula
See also
Normal Vector, Principal CurvaturesExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Rodrigues' Curvature Formula." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/RodriguesCurvatureFormula.html