Approximation by polynomials on quaternionic compact sets
  In this paper we obtain several extensions to the quaternionic setting of some results concerning the approximation by polynomials of functions continuous on a compact set and holomorphic in its interior. The results include approximation on compact starlike sets and compact axially symmetric sets. The cases of some concrete particular sets are described in details, including quantitative estimates too. 
Sorin G. Gal, Irene Sabadini
math.CV