Spectral Nevanlinna-Pick problem and weak extremals in the symmetrized   bidisc
  The main goal of the paper is to study $m$-extremal mappings in the symmetrized bidisc showing that they are rational and $\GG_2$-inner which, in particular, answers a question posed in \cite{Agl-Lyk-You 2013}. 
Lukasz Kosinski
math.CV math.FA