The quasi-potential approach is used to project a four-dimensional field-theoretical model of a three-particle system to the light-front (LF) valence sect or. The starting point is the Bethe-Salpeter equation (BSE) for three particles in the ladder approximation. The projection technique allows to eliminate three-body reducible diagrams from the kernel of the LF integral equation for the valence wave function. We construct the next-to-leading order contribution to the kernel of the LF three-body integral equation. We exemplify the method for a three-boson system with a contact interaction.
The repulsive delta-function interaction model in one dimension is reviewed for spinless particles and for spin-1/2 fermions. The problem of solving the differential equation related to the Schrödinger equation is reduced by the Bethe ansatz to a system of algebraic equations. The delta-function interaction is shown to have no effect on spinless fermions which therefore behave like free fermions, in agreement with Pauli's exclusion principle. The ground-state problem of spinless bosons is reduced to an inhomogeneous Fredholm equation of the second kind. In the limit of impenetrable interactions, the spinless bosons are shown to have the energy spectrum of free fermions. The model for spin-1/2 fermions is reduced by the Bethe ansatz to an eigenvalue problem of matrices of the same sizes as the irreducible representations R of the permutation group of N elements. For some R's this eigenvalue problem itself is solved by a generalized Bethe ansatz. The ground-state problem of spin-1/2 fermions is reduced to a generalized Fredholm equation.