Abstract: In 1974, a well-known research problem in Statistical Mechanics entitled "To determine and define the probability function P.sub.2 of a particle hitting a predesignated area, given all its parameters of generation and ejection." was openly solicited for its solution from research and development organizations in U.S.A. One of many proposed solutions of the problem, initiated at that time, is by means of the TRAJECTORY SOLID ANGLE(TSA). TSA is defined as the integral of the dot product of the unit tangent of the particle's trajectory to the vectorial area divided by the square of the position vector connecting between the point of ejection and that of the surface to be hit. The invention provides:(1) The precise and the unique solution of a previously unsolved P.sub.