  The Bogdanov-Takens system has at most one limit cycle and, in the parameter
space, it exists between a Hopf and a saddle-loop bifurcation curves. The aim
of this paper is to prove the Perko's conjectures about some analytic
properties of the saddle-loop bifurcation curve. Moreover, we provide sharp
piecewise algebraic upper and lower bounds for this curve.
