Quantum duality principle and quantum symmetric pairs

The quantum duality principal (QDP) by Drinfeld predicts a connection between the quantized universial enveloping algebras and the quantized coordinate algebras, where the underlying classical objects are related by the duality in Poisson geometry. The current paper gives an explicit formulization of the QDP for quantum symmetric pairs. Let $\mathfrak{g}$ be a complex semi-simple Lie algebra, equipped with the standard Lie bialgebra structure. Let $\theta$ be a Lie algebra involution on $\mathfrak{g}$ and denote by $\mathfrak{k}=\mathfrak{g}^\theta$ the fixed point subalgebra. The quantum symmetric pair $(\mathrm{U},\mathrm{U}^\imath)$ is originally defined to be a quantization of the symmetric pair of the universial enveloping algebras $(U(\mathfrak{g}),U(\mathfrak{k}))$. In this paper, we show that an explicit specialisation of $(\mathrm{U},\mathrm{U}^\imath)$ gives rise to the pair of the coordinate algebras $(\mathcal{O}(G^*),\mathcal{O}(K^\perp\backslash G^*))$, where $G^*$ is the dual Poisson-Lie group with the Lie algebra $\mathfrak{g}^*$, and $K^\perp\backslash G^*$ is a $G^*$-Poisson homogeneous space. Here $K^\perp$ is the closed subgroup of $G^*$associated to the complementary dual of $\mathfrak{k}$. Therefore $(\mathrm{U},\mathrm{U}^\imath)$ can be viewed as a pair of quantized coordinate algebras. This generalises the well-known fact that the quantum group $\mathrm{U}$ provides a quantization of the coordinate algebra $\mathcal{O}(G^*)$.