The $Pi$-operator, also known as Ahlfors-Beurling transform, plays an important role in solving the existence of locally quasiconformal solutions of Beltrami equations. In this paper, we first construct the $Pi$-operator on a general Clifford-Hilbert module. This $Pi$-operator is also an $L^2$ isometry. Further, it can also be used for solving certain Beltrami equations when the Hilbert space is the $L^2$ space of a measure space. Then, we show that this technique can be applied to construct the classical $Pi$-operator in the complex plane and some other examples on some conformally flat manifolds, which are constructed by $U/Gamma$, where $U$ is a simply connected subdomain of either $mathbb{R}^{n}$ or $mathbb{S}^{n}$, and $Gamma$ is a Kleinian group acting discontinuously on $U$. The $Pi$-operators on those manifolds also preserve the isometry property in certain $L^2$ spaces, and their $L^p$ norms are bounded by the $L^p$ norms of the $Pi$-operators on $mathbb{R}^{n}$ or $mathbb{S}^{n}$, depending on where $U$ lies. The applications of the $Pi$-operator to solutions of the Beltrami equations on those conformally flat manifolds are also discussed. At the end, we investigate the $Pi$-operator theory in the upper-half space with the hyperbolic metric.
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