Andradas and Recio, 2007) that the only point of C⋆ that may not be reached by the parametrization is P∞ = limt→±∞ φ(t), whenever it exists. In the sequel we will discard the isolated points of C⋆ (which come up from complex values of t) and we will focus on the remaining part of C⋆. Since polar coordinates provide a natural mapping Π (not 1:1) from C⋆ onto C, we will say that a point of C is reached by φ(t) if it has the form Π(P), where P ∈C⋆ has been reached by φ(t).
Now if C is either a circle centered at the origin or a line passing through the origin, then it is algebraic in both algebraic and polar forms at the same time (in fact, C⋆ is represented by an equation of the type r − r0 = 0 or θ − θ0 = 0). The next theorem proves that circles and lines are the only cases when this phenomenon happens. We acknowledge here the help of Fernando San Segundo for proving the theorem. Here we use the expression real curve to mean a curve with infinitely many real points.