Dehant, V., & Zschau, J. (1989). The Effect of Mantle Inelasticity On Tidal Gravity: A Comparison Between the Spherical and the Elliptical Earth Model. Geophysical Journal International, 97(3), 549-555. https://doi.org/10.1111/j.1365-246X.1989.tb00522.x (Original work published 1989)