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We study several problems related to the \(\ell ^p\) boundedness of Riesz transforms for graphs endowed with so-called bounded Laplacians. Introducing a proper notion of the gradient of a function, we prove for \(p\in (1,2]\) an \(\ell ^p\) estimate for the gradient of the continuous time heat semigroup, an \(\ell ^p\) interpolation inequality as well as the \(\ell ^p\) boundedness of the modified Littlewood–Paley–Stein function for a graph with bounded Laplacian. This yields an analogue to Dungey’s results in [21] while removing some additional assumptions. Coming back to the classical notion of the gradient, we give a counterexample to the interpolation inequality and hence to the boundedness of Riesz transforms for bounded Laplacians for \(1<p<2\). Finally, we prove the boundedness of the Riesz transform for \(1< p<\infty \) under the assumption of positive spectral gap.

Riesz transforms for bounded Laplacians on graphs