Deville, MO., Mund, EH., & Patera, AT. (1987). Iterative Solution of Isoparametric Spectral Element Equations By Low-order Finite-element Preconditioning. Journal of Computational and Applied Mathematics, 20, 189-197. https://doi.org/10.1016/0377-0427(87)90136-1 (Original work published 1987)