### Maximal estimates for nonsymmetric semigroups

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Let L be the full laplacian on the Heisenberg group ${\mathbb{H}}^{n}$ of arbitrary dimension n. Then for $f\in {L}^{2}\left({\mathbb{H}}^{n}\right)$ such that ${(I-L)}^{s/2}f\in {L}^{2}\left({\mathbb{H}}^{n}\right)$, s > 3/4, for a $\varphi \in {C}_{c}\left({\mathbb{H}}^{n}\right)$ we have ${\u0283}_{{\mathbb{H}}^{n}}\left|\varphi \left(x\right)\right|su{p}_{0<t\le 1}{\left|{e}^{(\surd -1)tL}f\left(x\right)\right|}^{2}dx\le {C}_{\varphi}\parallel f{\parallel}_{{W}^{s}}^{2}$. On the other hand, the above maximal estimate fails for s < 1/4. If Δ is the sublaplacian on the Heisenberg group ${\mathbb{H}}^{n}$, then for every s < 1 there exists a sequence ${f}_{n}\in {L}^{2}\left({\mathbb{H}}^{n}\right)$ and ${C}_{n}>0$ such that ${(I-L)}^{s/2}{f}_{n}\in {L}^{2}\left({\mathbb{H}}^{n}\right)$ and for a $\varphi \in {C}_{c}\left({\mathbb{H}}^{n}\right)$ we have ${\u0283}_{{\mathbb{H}}^{n}}\left|\varphi \left(x\right)\right|su{p}_{0<t\le 1}{\left|{e}^{(\surd -1)t\Delta}{f}_{n}\left(x\right)\right|}^{2}dx\ge {C}_{n}\parallel {f}_{n}{\parallel}_{{W}^{s}}^{2},li{m}_{n\to \infty}{C}_{n}=+\infty $.

We prove the dimension free estimates of the ${L}^{p}\to {L}^{p}$, 1< p ≤ ∞, norms of the Hardy-Littlewood maximal operator related to the optimal control balls on the Heisenberg group ℍⁿ.

Let G be the simplest nilpotent Lie group of step 3. We prove that the densities of the semigroup generated by the sublaplacian on G are not real-analytic.

We prove that on a product of generalized Heisenberg groups, a Hörmander type multiplier theorem for Rockland operators is true with the critical index n/2 + ϵ, ϵ>0, where n is the euclidean (topological) dimension of the group.

For a Schrödinger operator A = -Δ + V, where V is a nonnegative polynomial, we define a Hardy ${H}_{A}^{1}$ space associated with A. An atomic characterization of ${H}_{A}^{1}$ is shown.

Let A = -Δ + V be a Schrödinger operator on ${\mathbb{R}}^{d}$, d ≥ 3, where V is a nonnegative potential satisfying the reverse Hölder inequality with an exponent q > d/2. We say that f is an element of ${H}_{A}^{p}$ if the maximal function $su{p}_{t>0}\left|{T}_{t}f\left(x\right)\right|$ belongs to ${L}^{p}\left({\mathbb{R}}^{d}\right)$, where ${{T}_{t}}_{t>0}$ is the semigroup generated by -A. It is proved that for d/(d+1) < p ≤ 1 the space ${H}_{A}^{p}$ admits a special atomic decomposition.

Let ${{K}_{t}}_{t>0}$ be the semigroup of linear operators generated by a Schrödinger operator -L = Δ - V with V ≥ 0. We say that f belongs to $H{\xb9}_{L}$ if $\left|\right|su{p}_{t>0}|{K}_{t}{f\left(x\right)\left|\right||}_{L\xb9\left(dx\right)}<\infty $. We state conditions on V and ${K}_{t}$ which allow us to give an atomic characterization of the space $H{\xb9}_{L}$.

Let {T} be the semigroup of linear operators generated by a Schrödinger operator -A = Δ - V, where V is a nonnegative potential that belongs to a certain reverse Hölder class. We define a Hardy space H by means of a maximal function associated with the semigroup {T}. Atomic and Riesz transforms characterizations of H are shown.

A simple proof of the existence of solutions for the two-dimensional Keller-Segel model with measures with all the atoms less than 8π as the initial data is given. This result was obtained by Senba and Suzuki (2002) and Bedrossian and Masmoudi (2014) using different arguments. Moreover, we show a uniform bound for the existence time of solutions as well as an optimal hypercontractivity estimate.

Let L be a positive Rockland operator of homogeneous degree d on a graded homogeneous group G and let ${p}_{t}$ be the convolution kernels of the semigroup generated by L. We prove that if τ(x) is a Riemannian distance of x from the unit element, then there are constants c>0 and C such that $|{p}_{1}\left(x\right)|\le Cexp(-c\tau {\left(x\right)}^{d/(d-1)})$. Moreover, if G is not stratified, more precise estimates of ${p}_{1}$ at infinity are given.

For rank one solvable Lie groups of the type NA estimates for the Poisson kernels and their derivatives are obtained. The results give estimates on the Poisson kernel and its derivatives in a natural parametrization of the Poisson boundary (minus one point) of a general homogeneous, simply connected manifold of negative curvature.

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