In the case of an infinite reflector, the group diffusion equations are converted to a system of integral equations by a Green's function technique. This system is then Fourier transformed. The transformed Fredholm-type equations have a degenerate kernel, and the solution may be reduced to a system of linear algebraic equations with infinitely many unknowns. The theory is developed for the case of a three-group three-region cylindrical flux trap assembly. An extension of the method to a general multigroup multiregion problem will be considered in a later paper.