The curvature of a surface satisfies
 |
where 
is the normal curvature in a direction making an angle 
with the first principal direction and 
and 
are the principal
curvatures.
Euler Curvature Formula
See also
Principal CurvaturesExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Euler Curvature Formula." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/EulerCurvatureFormula.html