sentences:  The condition for the stationary value of the functional is that the functions formula_7 and formula_8 satisfy the Lippmann-Schwinger equation
  and
  
  Different form of the stationary principle for T-matrix reads
  The wave functions formula_7 and formula_8 must satisfy the same Lippmann-Schwinger equations to get the stationary value.The principle may be used for the calculation of the scattering amplitude in the similar way like the variational principle for bound states, i.e.the form of the wave functions formula_14 is guessed, with some free parameters, that are determined from the condition of stationarity of the functional.