1. Ashbaugh, M.S.: Isoperimetric and universal inequalities for eigenvalues. In: Spectral theory and geometry (Edinburgh, 1998), Davies, E.B., Yu Safalov (eds.), London Math. Soc. Lecture Notes, Vol. 273, Cambridge: Cambridge Univ. Press, 1999, pp. 95–139
2. Ashbaugh, M.S.: Universal eigenvalue bounds of Payne-Pólya-Weinberger, Hile-Protter and H.C. Yang. Proc. Indian Acad. Sci. Math. Sci. 112, 3–30 (2002)
3. Ashbaugh, M.S., Hermi, L.: A unified approach to universal inequalities for eigenvalues of elliptic operators, Pacific J. Math. 217, 3–30 (2004)
4. Chen, Z.-C., Qian, C.-L.: Estimates for discrete spectrum of Laplacian operator with any order. J. China Univ. Sci. Tech. 20, 259–266 (1990)
5. Cheng, Q.-M., Yang, H.C.: Estimates on Eigenvalues of Laplacian, Math. Ann. 331, 445–460 (2005)