Kurylev, Yaroslav, and Matti Lassas. “Multidimensional Gel’fand Inverse Boundary Spectral Problem: Uniqueness and Stability”. CUBO, A Mathematical Journal, vol. 8, no. 1, Apr. 2006, pp. 41–59, https://cubo.ufro.cl/index.php/cubo/article/view/1611.