Plane curves

A plane curve is the set of the form $\{(x,y):f(x,y)=0\}$where f(x,y) is a polynomial in two variables1. There are many familiar examples of plane curves: for example, the circle (x-3)2 + (y-2)2 = 4is a plane curve, as one sees by taking f(x,y) to be (x-3)2 + (y-2)2 - 4.

The degree of the curve is the total degree of f; this is defined as the maximum of i+j such that there is a monomial a xi yj occurring in f with $a \not=0$. For example, the plane curve

x3 - 10 x2 y2 + 9 y3 + 20 = 0

has degree 4 because of the monomial of largest degree in it is -10 x2 y2, which has degree 2+2=4.