Realizations of inner automorphisms of order four and fixed points subgroups by them on the connected compact exceptional Lie group $E_8$, Part III

Abstract

The compact simply connected Riemannian 4-symmetric spaces were classified by J.A. Jim{\'{e}}nez according to type of the Lie algebras. As homogeneous manifolds, these spaces are of the form $G/H$, where $G$ is a connected compact simple Lie group with an automorphism $\tilde{\gamma}$ of order four on $G$ and $H$ is a fixed points subgroup $G^\gamma$ of $G$. According to the classification by J.A. Jim{\'{e}}nez, there exist seven compact simply connected Riemannian 4-symmetric spaces $ G/H $ in the case where $ G $ is of type $ E_8 $. In the present article, %as Part II continuing from Part I, for the connected compact %exceptional Lie group $E_8$, we give the explicit form of automorphisms $\tilde{\omega}^{}_4,\tilde{\kappa}^{}_4$ and $ \tilde{\varepsilon}^{}_4$ of order four on $E_8$ induced by the $C$-linear transformations $\omega^{}_4, \kappa^{}_4$ and $ \varepsilon^{}_4$ of the 248-dimensional $ C $-vector space ${\mathfrak{e}_8}^{C}$, respectively. Further, we determine the structure of these fixed points subgroups $(E_8)^{{}_{\omega^{}_4}}, (E_8)^{{}_{\kappa^{}_4}}$ and $(E_8)^{{}_{\varepsilon^{}_4}}$ of $ E_8 $. These amount to the global realizations of three spaces among seven Riemannian 4-symmetric spaces $ G/H $ above corresponding to the Lie algebras $ \mathfrak{h}=\mathfrak{su}(2)\oplus i\mathbf{R} \oplus \mathfrak{e}_6, i\mathbf{R} \oplus \mathfrak{so}(14)$ and $\mathfrak{h} =\mathfrak{su}(2) \oplus i\mathbf{R}\oplus \mathfrak{so}(12)$, where $ \mathfrak{h}={\rm Lie}(H) $. With this article, the all realizations of inner automorphisms of order four and fixed points subgroups by them have been completed in $ E_8 $.