Solving the sextic by iteration: A complex dynamical approach

Scott Crass, Peter Doyle

Published 1999-03-17, updated 1999-03-18Version 2

Recently, Peter Doyle and Curt McMullen devised an iterative solution to the fifth degree polynomial. At the method's core is a rational mapping of the Riemann sphere with the icosahedral symmetry of a general quintic. Moreover, this map posseses "reliable" dynamics: for almost any initial point, the its trajectory converges to one of the periodic cycles that comprise an icosahedral orbit. This symmetry-breaking provides for a reliable or "generally-convergent" quintic-solving algorithm: with almost any fifth-degree equation, associate a rational mapping that has reliable dynamics and whose attractor consists of points from which one computes a root. An algorithm that solves the sixth-degree equation requires a dynamical system with the symmetry of the alternating group on six things. This group does not act on the Riemmann sphere, but does act on the complex projective plane--this is the Valentiner group. The present work exploits the resulting 2-dimensional geometry in finding a Valentiner-symmetric rational mapping whose elegant dynamics experimentally appear to be reliable in the above sense---transferred to the 2-dimensional setting. This map provides the central feature of a conjecturally-reliable sextic-solving algorithm analogous to that employed in the quintic case.