Dupont, S., & Marchal, JM. (1988). Preconditioned Conjugate Gradients for Solving the Transient Boussinesq Equations in 3-dimensional Geometries. International Journal for Numerical Methods in Fluids, 8(3), 283-303. https://doi.org/10.1002/fld.1650080303 (Original work published 1988)