Sharp $L^p$ estimates for discrete second order {R}iesz transforms

We show that multipliers of second order Riesz transforms on products of discrete abelian groups enjoy the $L^{p}$ estimate $p^{\ast} -1$, where $p^{\ast} = \max \{ p,q \}$ and $p$ and $q$ are conjugate exponents. This estimate is sharp if one considers all multipliers of the form $\sum_i σ_{i} R_{i} R^{\ast}_{i}$ with $| σ_{i} | \leqslant 1$ and infinite groups. In the real valued case, we obtain better sharp estimates for some specific multipliers, such as $\sum_{i} σ_{i} R_{i} R^{\ast}_{i}$ with $0 \leqslant σ_{i} \leqslant 1$. These are the first known precise $L^{p}$ estimates for discrete Calderón-Zygmund operators.

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