We consider the combined Walsh function for the three-dimensional case. A method for the solution of the neutron transport equation in three-dimensional case by using the Walsh function, Chebyshev polynomials, and the Legendre polynomials are considered. We also present Tau method, and it was proved that it is a good approximate to exact solutions. This method is based on expansion of the angular flux in a truncated series of Walsh function in the angular variable. The main characteristic of this technique is that it reduces the problems to those of solving a system of algebraic equations; thus, it is greatly simplifying the problem.
Abdelouahab Kadem. Adem Kilicman. "Note on the Solution of Transport Equation by Tau Method and Walsh Functions." Abstr. Appl. Anal. 2010 1 - 13, 2010. https://doi.org/10.1155/2010/704168