An algebraic curve is the subset of the plane formed by the solutions (x, y) of a non-constant polynomial equation in two variables, (19.1) C = {(x, y) ∈ R2 : f(x, y) = 0}. To make our language more precise, let’s say that xiyj is a monomial of degree i + j. Then, a polynomial of degree ≤ d is a sum of monomials.