Brovelli, M. A., and F. Migliaccio. “The Direct Estimation of the Potential Coefficients by Biorthogonal Sequences”. Annals of Geophysics 36, no. 5-6 (November 25, 1993). Accessed April 25, 2024. https://www.annalsofgeophysics.eu/index.php/annals/article/view/4242.