English

On pseudo-real finite subgroups of $\operatorname{PGL}_3(\mathbb{C})$

Group Theory 2023-09-19 v3

Abstract

Let GG be a finite subgroup of PGL3(C)\operatorname{PGL}_3(\mathbb{C}), and let σ\sigma be the generator of Gal(C/R)\operatorname{Gal}(\mathbb{C}/\mathbb{R}). We say that GG has a \emph{real field of moduli} if σG^{\sigma}G and GG are PGL3(C)\operatorname{PGL}_3(\mathbb{C})-conjugates, that is, if ϕPGL3(C)\exists\,\phi\in\operatorname{PGL}_3(\mathbb{C}) such that ϕ1Gϕ=σG\phi^{-1}\,G\,\phi=\,^{\sigma}G. Furthermore, we say that R\mathbb{R} is \emph{a field of definition for GG} or that \emph{GG is definable over R\mathbb{R}} if GG is PGL3(C)\operatorname{PGL}_3(\mathbb{C})-conjugate to some GPGL3(R)G'\subset\operatorname{PGL}_3(\mathbb{R}). In this situation, we call GG' \emph{a model for GG over R\mathbb{R}}. If GG has R\mathbb{R} as a field of definition but is not definable over R\mathbb{R}, then we call GG \emph{pseudo-real}. In this paper, we first show that any finite cyclic subgroup G=Z/nZG=\mathbb{Z}/n\mathbb{Z} in PGL3(C)\operatorname{PGL}_3(\mathbb{C}) has {a real field of moduli} and we provide a necessary and sufficient condition for G=Z/nZG=\mathbb{Z}/n\mathbb{Z} to be definable over R\mathbb{R}; see Theorems 2.1, 2.2, and 2.3. We also prove that any dihedral group D2n\operatorname{D}_{2n} with n3n\geq3 in PGL3(C)\operatorname{PGL}_3(\mathbb{C}) is definable over R\mathbb{R}; see Theorem 2.4. Furthermore, we study all six classes of finite primitive subgroups of PGL3(C)\operatorname{PGL}_3(\mathbb{C}), and show that all of them except the icosahedral group A5\operatorname{A}_5 are pseudo-real; see Theorem 2.5, whereas A5\operatorname{A}_5 is definable over R\mathbb{R}. Finally, we explore the connection of these notions in group theory with their analogues in arithmetic geometry; see Theorem 2.6 and Example 2.7.

Keywords

Cite

@article{arxiv.2301.00543,
  title  = {On pseudo-real finite subgroups of $\operatorname{PGL}_3(\mathbb{C})$},
  author = {Eslam Badr and Ahmad El-Guindy},
  journal= {arXiv preprint arXiv:2301.00543},
  year   = {2023}
}

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11 pages