The problem of coexistence of several non-Hermitian observables in   PT-symmetric quantum mechanics
  During the recent developments of quantum theory it has been clarified that the observable quantities (like energy or position) may be represented by operators (with real spectra) which are manifestly non-Hermitian. The mathematical consistency of the resulting models of stable quantum systems requires a reconstruction of an alternative, amended, physical inner product of states. We point out the less known fact that for more than one observable the task is not always feasible. The difficulty is re-analyzed and its elementary linear-algebraic interpretation and treatment are outlined. 
Miloslav Znojil, Iveta Semor\'adov\'a, Franti\v{s}ek   R\u{u}\v{z}i\v{c}ka, Hafida Moulla and Ilhem Leghrib
quant-ph math-ph math.MP