A tangent vector is a principal vector iff
where , , and are coefficients of the first fundamental form and , , of the second fundamental form.
A tangent vector is a principal vector iff
where , , and are coefficients of the first fundamental form and , , of the second fundamental form.
Weisstein, Eric W. "Principal Vector." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PrincipalVector.html