Richard S. Varga, Numerical methods for solving multi-dimensional multigroup diffusion equations, Proceedings of Symposia in Applied Mathematics, Nuclear Reactor Theory, 11, pp. 164- 189. Amer. Math. Soc. , Providence, RI, 1961. MR 23, 3595. 

1. Introduction. It is well known [48] that numerical solutions of multigroup diffusion equations have played, and will continue to play, an important part in the design of nuclear reactors. With the availability of digital computing machines with ever-increasing speed, the scope of nuclear design problems which can be numerically attacked with the diffusion approximation has also been increasing, bringing forth interesting new questions for numerical analysts.