Landen transformations applied to approximation

We study computational methods for the approximation of special functions recurrent in geometric function theory and quasiconformal mapping theory. The functions studied can be expressed as quotients of complete elliptic integrals and as inverses of such quotients. In particular, we consider the distortion function ${\phi }_K(r)$ which gives a majorant for $\mathrm{\mid }f(x)\mathrm{\mid }$ when $f:{\mathbb{B}}^2\to {\mathbb{B}}^2,f(0)=0,$ is a quasiconformal mapping of the unit disk ${\mathbb{B}}^2\mathrm{.}$ It turns out that the approximation method is very simple: five steps of Landen iteration is enough to achieve machine precision.