Haldenwang, P., Labrosse, G., Abboudi, S., & Deville, M. (1984). Tschebyschev 3-d Spectral and 2-d Pseudospectral Solvers for the Helmholtz-equation. Journal of Computational Physics, 55(1), 115-128. https://doi.org/10.1016/0021-9991(84)90018-4 (Original work published 1984)