Mean Value Conjectures for Rational Maps

Let p be a polynomial in one complex variable. Smale’s mean value conjecture estimates |p′(z)| in terms of the gradient of a chord from (z, p(z)) to some stationary point on the graph of p. The conjecture does not immediately generalise to rational maps since its formulation is invariant under the group of affine maps, not the full Möbius group. Here we give two possible generalisations to rational maps, both of which are Möbius invariant. In both cases we prove a version with a weaker constant, in parallel to the situation for Smale’s mean value conjecture. Finally, we discuss some candidate extremal rational maps, namely rational maps all of whose critical points are fixed points.