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Let $N$ be an $H$-type group and $S\simeq N\times {ℝ}^{+}$ be its harmonic extension. We study a left invariant Hardy–Littlewood maximal operator $M_{\rho }^{ℛ}$ on $S$, obtained by taking maximal averages with respect to the right Haar measure over left-translates of a family $ℛ$ of neighbourhoods of the identity. We prove that the maximal operator $M_{\rho }^{ℛ}$ is of weak type $(1,1)$.

Questa è una rassegna di alcuni risultati recenti su spazi di Hardy nel contesto di gruppi di Lie a crescita esponenziale di volume, che ho presentato nella conferenza da me tenuta a Bologna in occasione del XIX Congresso dell'Unione Matematica Italiana. Faremo un breve cenno alla teoria degli spazi di Hardy in ambito euclideo e al ruolo svolto da tali spazi nell'analisi armonica su ${ℝ}^n$. La parte cruciale della nostra presentazione consisterà nell'introduzione di una nuova teoria di spazi di Hardy...

Let $G$ be the Lie group ${ℝ}^2⋉{ℝ}^{+}$ endowed with the Riemannian symmetric space structure. Let $X_0,X_1,X_2$ be a distinguished basis of left-invariant vector fields of the Lie algebra of $G$ and define the Laplacian $\Delta =-(X_0^2+X_1^2+X_2^2)$. In this paper we consider the first order Riesz transforms $R_i=X_i{\Delta }^{-1/2}$ and $S_i={\Delta }^{-1/2}{X}_i$, for $i=0,1,2$. We prove that the operators $R_i$, but not the $S_i$, are bounded from the Hardy space $H^1$ to $L^1$. We also show that the second-order Riesz transforms $T_{ij}=X_i{\Delta }^{-1}{X}_j$ are bounded from $H^1$ to $L^1$, while the transforms $S_{ij}={\Delta }^{-1}{X}_i{X}_j$ and $R_{ij}=X_i{X}_j{\Delta }^{-1}$, for $i,j=0,1,2$, are not.

We consider a complete connected noncompact Riemannian manifold M with bounded geometry and spectral gap. We prove that the imaginary powers of the Laplacian and the Riesz transform are bounded from the Hardy space X¹(M), introduced in previous work of the authors, to L¹(M).