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

TL;DR

This paper explicitly constructs automorphisms of order four on the exceptional Lie group E8 and determines their fixed point subgroups, providing realizations of certain 4-symmetric spaces associated with E8.

Contribution

It explicitly describes automorphisms of order four on E8 and determines their fixed point subgroups, advancing the understanding of 4-symmetric spaces related to E8.

Findings

Explicit forms of automorphisms of order four on E8

Determination of fixed point subgroup structures

Realization of three specific 4-symmetric spaces

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 $\tilde{w}_{{}_4} \tilde{\upsilon}_{{}_4}$ and $\tilde{\mu}_{{}_4}$ of order four on $E_8$ induced by the $C$-linear transformations $w_{{}_4}, \upsilon_{{}_4}$ and $\mu_{{}_4}$ of the 248-dimensional vector space ${\mathfrak{e}_8}^{C}$, respectively. Further, we determine the structure of these fixed points subgroups $(E_8)^{w_{{}_4}}, (E_8)^{{}_{\upsilon_{{}_4}}}$ and $(E_8)^{{} _{\mu_{{}_4}}}$ 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}=i\bm{R} \oplus \mathfrak{su}(8), i\bm{R} \oplus \mathfrak{e}_7$ and $\mathfrak{h}= \mathfrak{su}(2) \oplus \mathfrak{su}(8)$, where $ \mathfrak{h}={\rm Lie}(H) $.