Geometrical Methods for Determining the Accuracy of Approximate Solutions of the Boltzmann Equation Part I. One-Group Scattering and Absorbing Media R. Ackroyd Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11044 10.1512/iumj.1962.11.11044 en Indiana Univ. Math. J. 11 (1962) 811 - 850 state-of-the-art mathematics http://iumj.org/access/