Cwikel's bound reloaded

TL;DR

This paper revisits Cwikel's proof of the CLR bound, offering a simplified approach that significantly improves the constant estimates and connects the bound with harmonic analysis techniques.

Contribution

It provides a simplified and more effective proof of Cwikel's approach, yielding better constants and extending to generalized Schrödinger operators.

Findings

New simplified proof improves constant estimates in CLR bound.

Establishes a connection between CLR bounds and harmonic analysis.

Extends bounds to a broader class of Schrödinger-type operators.

Abstract

There are a couple of proofs by now for the famous Cwikel--Lieb--Rozenblum (CLR) bound, which is a semiclassical bound on the number of bound states for a Schr\"odinger operator, proven in the 1970s. Of the rather distinct proofs by Cwikel, Lieb, and Rozenblum, the one by Lieb gives the best constant, the one by Rozenblum does not seem to yield any reasonable estimate for the constants, and Cwikel's proof is said to give a constant which is at least about 2 orders of magnitude off the truth. This situation did not change much during the last 40+ years. It turns out that this common belief, i.e, Cwikel's approach yields bad constants, is not set in stone: We give a drastic simplification of Cwikel's original approach which leads to an astonishingly good bound for the constant in the CLR inequality. Our proof is also quite flexible and leads to rather precise bounds for a large class of Schr\"odinger-type operators with generalized kinetic energies. Moreover, it highlights a natural but overlooked connection of the CLR bound with bounds for maximal Fourier multipliers from harmonic analysis.