If and then, and this means that is different from the null vector field.
Can anyone check I have calculated correctly the cross product of 2 vectors..
It is part of a problem to solve a quasi linear equation in which a horizontal plane cuts a sphere.
I am to check that the 2 functions phi and psi are linearly independent if there cross product is not equal to 0. Although it a very simple problem, I havent come across it before.