A Sharp Higher Order Sobolev Inequality on Riemannian Manifolds

TL;DR

This paper establishes a sharp higher-order Sobolev inequality on closed Riemannian manifolds, extending previous results for lower orders and identifying the optimal constant in the inequality.

Contribution

It proves a sharp higher-order Sobolev inequality on Riemannian manifolds for all relevant orders, generalizing earlier work for first and second orders.

Findings

Existence of a constant B depending on (M,g), m, n

The inequality is sharp with the best constant K(m,n)

Extension of previous results for m=1 and m=2

Abstract

Let $ m, n $ be integers such that $ \frac{n}{2} > m \geq 1 $ and let $ (M, g) $ be a closed $ n-$dimensional Riemannian manifold. We prove there exists some $ B \in \mathbb{R} $ depending only on $ (M, g) $, $ m $, and $ n $ such that for all $ u \in H_m^2(M) $, $$ \lVert u \rVert_{2^\#}^2 \leq K(m,n) \int_M (\Delta^\frac{m}{2} u)^2 dv_g + B \lVert u \rVert_{H_{m-1}^2(M)}^2 $$ where $ 2^\# = \frac{2n}{n-2m} $, $ K(m,n) $ is the square of the best constant for the embedding $ W^{m,2}(\mathbb{R}^n) \subset L^{2^\#}(\mathbb{R}^n) $, $ H_m^2(M) $ is the Sobolev space consisting of functions on $ M $ with $ m $ weak derivatives in $ L^2(M) $, and $ \Delta^\frac{m}{2} = \nabla \Delta^{\frac{m-1}{2}} $ if $ m $ is odd. This inequality is sharp in the sense that $ K(m,n) $ cannot be lowered to any smaller constant. This extends the work of Hebey-Vaugon and Hebey which correspond respectively to the cases $ m=1 $ and $ m=2 $.