“…One of the most central problems is to establish Payne-Pólya-Weinberger-Yang-type inequalities for the eigenvalues of the problem ∆ 2 g u = Γu in Ω, u = ∂u ∂n = 0 on ∂Ω, (1.4) where Ω is a bounded domain in an n-dimensional Riemannian manifold (M, g), ∆ 2 g stands for the biharmonic Laplace-Beltrami operator on (M, g) and ∂ ∂n is the outward normal derivative on ∂Ω, respectively; see e.g. Chen, Zheng and Lu [7], Cheng, Ichikawa and Mametsuka [8], Cheng and Yang [9,10,11], Wang and Xia [39]. Instead of (1.2), one naturally considers the fundamental tone of Ω ⊂ M by Γ g (Ω) := Γ g,n (Ω) = inf u∈W 2,2 0 (Ω)\{0} Ω (∆ g u) 2 dv g Ω u 2 dv g , (1.5) where dv g denotes the canonical measure on (M, g), and W 2,2 0 (Ω) is the usual Sobolev space on (M, g), see Hebey [19]; in fact, it turns out that Γ g (Ω) is the first eigenvalue to (1.4).…”