The Hardy-Rellich inequality and uncertainty principle on the sphere

Abstract: Let $\Delta_0$ be the Laplace-Beltrami operator on the unit sphere $\mathbb{S}^{d-1}$ of $\mathbb{R}^d$. We show that the Hardy-Rellich inequality of the form $$ \int_{\mathbb{S}^{d-1}} \left | f (x)\right|^2 d\sigma(x) \leq c_d \min_{e\in \mathbb{S}^{d-1}} \int_{\mathbb{S}^{d-1}} (1- \langle x, e \rangle) \left |(-\Delta_0)^{\frac{1}{2}}f(x) \right |^2 d\sigma(x) $$ holds for $d =2$ and $d \ge 4$ but does not hold for $d=3$ with any finite constant, and the optimal constant for the inequality is $c_d = 8/(d-3)^2$ for $d =2, 4, 5$ and, under additional restrictions on the function space, for $d\ge 6$. This inequality yields an uncertainty principle of the form $$ \min_{e\in\mathbb{S}^{d-1}} \int_{\mathbb{S}^{d-1}} (1- \langle x, e \rangle) |f(x)|^2 d\sigma(x) \int_{\mathbb{S}^{d-1}}\left |\nabla_0 f(x)\right |^2 d\sigma(x) \ge c'_d $$ on the sphere for functions with zero mean and unit norm, which can be used to establish another uncertainty principle without zero mean assumption, both of which appear to be new. This paper is published in Constructive Approximation, 40(2014): 141-171. An erratum is now appended.