Uniform Sobolev Resolvent Estimates for the Laplace–Beltrami Operator on Compact Manifolds

In this paper, we continue the study on the resolvent estimates of the Laplace–Beltrami operator Δg on a compact manifold M with dimension n≥3. On the Sobolev line 1/p−1/q=2/n, we can prove that the resolvent (Δg+ζ)⁻¹ is uniformly bounded from L^p to L^q when (p,q) are within the range: p≤2(n+1)/(n+3) and q≥2(n+1)/(n−1) and ζ is outside a parabola opening to the right and a small disk centered at the origin. This naturally generalizes the previous results in [2, 3] which addressed only the special case when p=2n/(n+2),q=2n/(n−2). Using the shrinking spectral estimates between L^p and L^q, we also show that when (p,q) are within the interior of the range mentioned above, one can obtain a logarithmic improvement over the parabolic region for resolvent estimates on manifolds equipped with Riemannian metric of nonpositive sectional curvature, and a power improvement depending on the exponent (p,q) for flat torus. The latter therefore partially improves Shen's work in [5] on the L^p→L² uniform resolvent estimates on the torus. Similar to the case as proved in [2] when (p,q)=(2n/(n+2),2n/(n−2)), the parabolic region is also optimal over the round sphere Sⁿ when (p,q) are now in the range. However, we may ask whether the range is sharp in the sense that it is the only possible range on the Sobolev line for which a compact manifold can have uniform resolvent estimate for ζ being outside a parabola.