Super-Poincaré and Nash-type inequalities for Subordinated Semigroups

Hypercontractivity, Nash inequalities, and subordination for classes of nonlinear semigroups

A suitable notion of hypercontractivity for a nonlinear semigroup $\{T_t\}$ is shown to imply Gagliardo--Nirenberg inequalities for its generator $H$, provided a subhomogeneity property holds for the energy functional $(u,Hu)$. We use this fact to prove that, for semigroups generated by operators of $p$--Laplacian--type, hypercontractivity implies ultracontractivity. We then introduce the notion of subordinated nonlinear semigroups when the corresponding Bernstein function is $f(x)=x^α$, and write down an explicit formula for the associated generator. It is shown that, in certain cases, hyperc…

Non ultracontractive heat kernel bounds by Lyapunov conditions

Nash and Sobolev inequalities are known to be equivalent to ultracontractive properties of heat-like Markov semigroups, hence to uniform on-diagonal bounds on their kernel densities. In non ultracontractive settings, such bounds can not hold, and (necessarily weaker, non uniform) bounds on the semigroups can be derived by means of weighted Nash (or super-Poincaré) inequalities. The purpose of this note is to show how to check these weighted Nash inequalities in concrete examples, in a very simple and general manner. We also deduce off-diagonal bounds for the Markov kernels of the semigroups, r…

Weak Poincaré inequalities in the absence of spectral gaps

For generators of Markov semigroups which lack a spectral gap, it is shown how bounds on the density of states near zero lead to a so-called "weak Poincaré inequality" (WPI), originally introduced by Liggett [Ann. Probab., 1991]. Applications to general classes of constant coefficient pseudodifferential operators are studied. Particular examples are the heat semigroup and the semigroup generated by the fractional Laplacian in the whole space, where the optimal decay rates are recovered. Moreover, the classical Nash inequality appears as a special case of the WPI for the heat semigroup.

Heisenberg群上的对数Sobolev、Hardy与Poincaré不等式

Logarithmic Sobolev, Hardy and Poincaré inequalities on the Heisenberg group

本文在Heisenberg群上系统证明含显式常数的Sobolev、Hardy、Nash、对数Gross及广义Poincaré等不等式，并实现无限维推广。

椭圆哈纳克不等式对于混合局部和非局部 $p$-能量形式在度量测度空间中的应用

Elliptic Harnack inequalities for mixed local and nonlocal $p$-energy form on metric measure spaces

本文在度量测度空间中提出混合局部和非局部 $p$-能量形式的公理化框架，并利用泊松不等式和截断塞伯里格不等式建立了弱和强椭圆哈纳克不等式，扩展了狄利克雷形式和欧几里得空间上混合能量形式的结果。