  Let $P$ be an arbitrary point on an elliptic curve over the complex numbers
of the form $y^2=x^3+a_4\,x+a_6$ or of the form $y^2=x^3+a_2\,x^2+a_4\,x$. We
provide explicit formulae to compute the points $P/2$, i.e., the points $Q$
such that $2*Q=P$.
