GUO, Shunzi. Operator $\Box^{r}$ on a submanifold of Riemannian manifold and its applications. Armenian Journal of Mathematics, Yerevan, Armenia, v. 7, n. 1, p. 6–31, 2015. Disponível em: https://armjmath.sci.am/index.php/ajm/article/view/108. Acesso em: 3 apr. 2025.