Christoph Börgers Edward W. Larsen The Transversely Integrated Scalar Flux of a Narrowly Focused Particle Beam. 1-22 1995 55 SIAM J. Appl. Math. 1 https://doi.org/10.1137/S0036139993252293 db/journals/siamam/siamam55.html#BorgersL95
