In electromagnetism we have these two Lorentz scalars: ##P=B^2-E^2## ##Q=E\cdot B## WP https://en.wikipedia.org/wiki/Classification_of_electromagnetic_fields claims that these are a complete set of invariants, because "every other invariant can be expressed in terms of these two." How does one prove this? Would the idea be to show that any electromagnetic field tensor can be rendered into one of a set of canonical forms by boosts and rotations? Or maybe you could fiddle with the eigenvalues of the field tensor? Is the claim only true for invariants that are continuous functions of the field tensor (i.e., continuous functions of its components)?