Abstract We improve the work [R. L. Frank and J. Sabin, Spectral cluster bounds for orthonormal systems and oscillatory integral operators in Schatten spaces, Adv. Math. 317 2017, 157–192] concerning the spectral cluster bounds for orthonormal systems at p = ∞ {p=\infty} , on the flat torus and spaces of nonpositive sectional curvature, by shrinking the spectral band from [ λ 2 , ( λ + 1 ) 2 ) {[\lambda^{2},(\lambda+1)^{2})} to [ λ 2 , ( λ + ϵ ⁢ ( λ ) ) 2 ) {[\lambda^{2},(\lambda+\epsilon(\lambda))^{2})} , where ϵ ⁢ ( λ ) {\epsilon(\lambda)} is a function of λ that goes to 0 as λ goes to ∞ {\infty} . In achieving this, we invoke the method developed in [J. Bourgain, P. Shao, C. D. Sogge and X. Yao, On L p L^{p} -resolvent estimates and the density of eigenvalues for compact Riemannian manifolds, Comm. Math. Phys. 333 2015, 3, 1483–1527].

The carré du champ method is a powerful technique for proving interpolation inequalities with explicit constants in presence of a non-trivial metric on a manifold. The method applies to some classical Gagliardo-Nirenberg-Sobolev inequalities on the sphere, with optimal constants. Very nonlinear regimes close to the critical Sobolev exponent can be covered using nonlinear parabolic flows of porous medium or fast diffusion type. Considering power law weights is a natural question in relation with symmetry breaking issues for Caffarelli-Kohn-Nirenberg inequalities, but regularity estimates for a complete justification of the computation are missing. We provide the first example of a complete parabolic proof based on a nonlinear flow by regularizing the singularity induced by the weight. Our result is established in the simplified framework of a diffusion built on the ultraspherical operator, which amounts to reduce the problem to functions on the sphere with simple symmetry properties.

The authors get on Métivier groups the spectral resolution of a class of operators Open image in new window , the joint functional calculus of the sub-Laplacian and Laplacian on the centre, and then give some restriction theorems together with their asymptotic estimates, asserting the mix-norm boundedness of the spectral projection operators \(\mathcal{P}_\mu^m\) for two classes of functions m(a, b) = (aα + bβ)γ or (1 + aα + bβ)γ, with α, β > 0, γ ≠ 0.

This paper collects results concerning global rates and large time asymptotics of a fractional fast diffusion equation on the Euclidean space, which is deeply related with a family of fractional Gagliardo–Nirenberg–Sobolev inequalities. Generically, self-similar solutions are not optimal for the Gagliardo–Nirenberg–Sobolev inequalities, in strong contrast with usual standard fast diffusion equations based on non-fractional operators. Various aspects of the stability of the self-similar solutions and of the entropy methods like carré du champ and Rényi entropy powers methods are investigated and raise a number of open problems.

This paper is devoted to optimal functional inequalities for fractional Laplace operators on the sphere. Based on spectral properties, subcritical inequalities are established. Their consequences for fractional heat flows are considered. These subcritical inequalities interpolate between fractional Sobolev and subcritical fractional logarithmic Sobolev inequalities. Their optimal constants are determined by a spectral gap. In the subcritical range, the method also provides us with remainder terms which can be considered as an improved version of the optimal inequalities. We also consider inequalities which interpolate between fractional logarithmic Sobolev and fractional Poincar{é} inequalities. Finally, weighted inequalities involving the fractional Laplacian are obtained in the Euclidean space, using a stereographic projection and scaling properties.

In this paper, we got several sharp Hardy-Littlewood-Sobolev-type inequalities on quaternionic Heisenberg groups (a general form due to Folland and Stein [FS74]), using the symmetrization-free method in a paper of Frank and Lieb [FL12], where they considered the analogues on classical Heisenberg group. First, we give the sharp Hardy-Littlewood-Sobolev inequalities, both on quaternionic Heisenberg group and its equivalent on quaternionic sphere for exponent bigger than 4. The extremizer, as we guess, is almost uniquely constant function on sphere. Then their dual form, sharp conformally-invariant Sobolev inequalities and the right endpoint limit case, Log-Sobolev inequality, are also obtained. For small exponent less 4, constant function is only proved to be a local extremizer. The conformal symmetry of the inequalities and zero center-mass technique play a critical role in the argument.

This report is based on a talk given by the author in the Laurent Schwartz seminar at IHÉS, Paris, on February 16, 2016. This involves joint works with Michael Christ and Heping Liu [CLZ16a, CLZ16b, LZ15]. We review several sharp Hardy-Littlewood-Sobolev-type inequalities (HLS) on I-type groups (rank one), which is a special class of H-type groups, using the symmetrization-free method of Frank and Lieb, who proved the sharp HLS on the Heisenberg group in a seminal paper [FL12b]. We give the sharp HLS both on the compact and noncompact pictures. The “unique” extremal function, as expected, can only be constant function on the sphere. Their dual form, a sharp conformally invariant inequality involving an intertwining operator (“fractional subLaplacian”), and the right endpoint case, a Log-Sobolev inequality, are also obtained. Besides, some stability and dual type improvements are also discussed. A positivity-type restriction on the singular exponent is required in the cases with centres of high dimensions, which bring extra difficulty. The conformal symmetry of the inequalities, zero center-mass technique, estimates involving meticulous computation of eigenvalues of singular kernels, compactness and local stability play a critical role in the argument.

We give estimates of the remainder terms for several conformally-invariant Sobolev-type inequalities on the Heisenberg group. By considering the variations of associated functionals, we give a stability for two dual inequalities: The fractional Sobolev(FS) and Hardy-Littlewood-Sobolev(HLS) inequalities, in terms of distance to the submanifold of extremizers. Then we compare their remainder terms to improve the inequalities in another way. We also compare, in the limit case, the remainder terms of Beckner-Onofri(BO) inequality and its dual logarithmic Hardy-Littlewood-Sobolev(Log-HLS) inequality. Besides, we also list without proof some results for other groups of Iwasawa-type. Our results generalize earlier works on Euclidean spaces of Chen et al.(2013) and Dolbeault and Jankowiak(2014) onto some groups of Heisenberg-type. We worked for "almost"all fractions especially for comparing results, and the stability of HLS is also absolutely new, even for Euclidean case.