Department of Mathematics, Hunan Normal University, Changsha, Hunan 410081, People's Republic of China ; Department of Mathematics, Tsinghua University, Beijing 100080, People's Repulic of China
Abstract:
Let be a polynomial semigroup containing an element with degree at least 2 with the semigroup operation being functional composition. We prove that is nearly abelian if and only if the smallest completely invariant closed subset of the Riemann sphere is not equal to the Riemann sphere. We also give a positive answer to Conjecture 7.1 in Hinkkanen and Martin's paper on the dynamics of semigroups of rational functions.