Lamberti, P.D., and L. Provenzano. “A Maximum Principle in Spectral Optimization Problems for Elliptic Operators Subject to Mass Density Perturbations”. Eurasian Mathematical Journal 4, no. 3 (September 30, 2013): 70–83. Accessed October 31, 2025. https://emj.enu.kz/index.php/main/article/view/538.