Fixed Space of Positive Trace-Preserving Super-Operators
  We examine the fixed space of positive trace-preserving super-operators. We describe a specific structure that this space must have and what the projection onto it must look like. We show how these results, in turn, lead to an alternative proof of the complete characterization of the fixed space of completely positive trace-preserving super-operators. 
Ansis Rosmanis
quant-ph