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

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) $.