Off-shell amplitudes for nonoriented closed strings
  In the context of the bosonic closed string theory, by using the operatorial formalism, we give a simple expression of the off-shell amplitude with an arbitrary number of external massless states inserted on the Klein bottle. 
L. Cappiello, R. Marotta, R. Pettorino and F. Pezzella (Dipartimento   di Scienze Fisiche, Universita' di Napoli and I.N.F.N...