sentences:  The functional is stationary if and only if the two functions satisfy the Lippmann-Schwinger equation.The development of the variational formulation of the scattering theory can be traced to works of L. Hultén and J. Schwinger in 1940s.The T-matrix expressed in the form of stationary value of the functional reads
  where formula_2 and formula_3 are the initial and the final states respectively, formula_4 is the interaction potential and formula_5 is the retarded Green's operator for collision energy formula_6.