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An elementary proof of the continuity from $L_0^2(Ω)$ to $H^1_0(Ω)^n$ of Bogovskii's right inverse of the divergence

The existence of right inverses of the divergence as an operator form $H^1_0(Ω)^n$ to $L_0^2(Ω)$ is a problem that has been widely studied because of its importance in the analysis of the classic equations of fluid dynamics. When $Ω$ is a bounded domain which is star-shaped with respect to a ball $B$, a right inverse given by an integral operator was introduced by Bogovskii, who also proved the continuity using the Calderón-Zygmund theory of singular integrals. In this paper we give an alternative elementary proof using the Fourier transform. As a consequence, we obtain estimates of the constant in the continuity in terms of the ratio between the diameters of $Ω$ and $B$. Moreover, using the relation between the existence of right inverses of the divergence with the Korn and improved Poincaré inequalities, we obtain estimates for the constants in these two inequalities.