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Published online by Cambridge University Press: 17 February 2025
Let G be the Lie group ${\mathbb{R}}^2\rtimes {\mathbb{R}}^+$ endowed with the Riemannian symmetric space structure. Take a distinguished basis
$X_0,\, X_1,\,X_2$ of left-invariant vector fields of the Lie algebra of G, and consider the Laplacian
$\Delta=-\sum_{i=0}^2X_i^2$ and the first-order Riesz transforms
$\mathcal R_i=X_i\Delta^{-1/2}$,
$i=0,1,2$. We first show that the atomic Hardy space H1 in G introduced by the authors in a previous paper does not admit a characterization in terms of the Riesz transforms
$\mathcal R_i$. It is also proved that two of these Riesz transforms are bounded from H1 to H1.