One-dimensional motion of three interacting particles is considered in the case when two of them form a bound state (compound particle). For the particular case when the excitation energy of the compound particle grows quadratically the effect of the infinite number of virtual excitations is investigated. It is shown that the population of the higher states is in inverse proportion to the cube of the excitation energy and the neglect of the states higher than the s^th one leads to an error of the order 1/8³. Besides, it is proved that the system of equations which takes into account all the virtual excitations of the compound particle can be solved by the Fredholm method.