The problem of positivity in 1+1 dimensions and Krein spaces
  The possibility of introducing a positive metric on the states of the massless scalar field in 1+1 dimensions by mean of Krein spaces is examined. Two different realisations in Krein spaces for the massless scalar field are compared. It is proved that one is a particular case of the other. The peculiarities and advantages of both realisations are discussed. 
Vera Montalbano
math-ph hep-th math.MP