When is a plane algebraic curve closed?
Is there a metric? For quadratic http://mathworld.wolfram.com/QuadraticCurveDiscriminant.html
we know the discriminant has to be below 0.
Is there such a thing for higher degrees? I'm not looking for suggestions. I'm looking for complete answers. Not proofs of existence, because I know it exists.
Do we know it and what is it?