[en] The methods are discussed in detail for the solution of the non-linear equation satisfied by the probability, as a function of neutron position, energy and moving direction, of a source neutron sponsoring a persistent fission chain in a system whose configuration does not change with time. The differences between the effectiveness coefficient of a neutron source in the sense of its sponsoring persistent fission chains and the effectiveness coefficient of the neutron source in the sense of neutron multiplication are also discussed. (authors)