Fabricant, Alexander, Nikolai Kutev, and Tsviatko Rangelov. “On the First Eigenvalue for Linear Second Order Elliptic Equations in Divergence Form”. CUBO, A Mathematical Journal 9, no. 3 (December 1, 2007): 47–64. Accessed December 7, 2025. https://cubo.ufro.cl/index.php/cubo/article/view/1587.