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Classification of finite subgroups of $\textrm{PGL}_2(\bar{\mathbb{F}}_p)$

Content of this project

This project contains the $\LaTeX$ blueprint and the (ongoing) Lean formalization of the classification of finite subgroups of $\textrm{PGL}_2(F)$.

Which states:

  • If $H$ is finite subgroup of $\textrm{PGL}_2(\mathbb{C})$ then $H$ is isomorphic to one of the following groups: the cyclic group $C_n$ of order $n$ ($n \in Z_{>0}$), the dihedral group $D_{2n}$ of order $2n$ ($n \in Z_{>1}$), $A_4$, $S_4$ or $A_5$.
  • If $H$ is a finite subgroup of $\textrm{PGL}_2(\bar{F}_p)$ then one of the following holds:
    1. $H$ is conjugate to a subgroup of the upper triangular matrices;
    2. $H$ is conjugate to $\textrm{PGL}_2 (F_{\ell^r})$ and $\textrm{PSL}_2 (F_{\ell^r})$ for some $r \in Z_{>0}$;
    3. $H$ is isomorphic to $A_4$, $S_4$, $A_5$ or the dihedral group $D_{2r}$ of order $2r$ for some $r \in Z_{>1}$ not divisible by $\ell$

Where $\ell$ is assumed to be an odd prime.

The big picture

This project is a contribution towards the formalization of Fermat's Last Theorem (see https://github.com/ImperialCollegeLondon/FLT), the result formalized covers Theorem 2.47 in FermatLastTheorem

Acknowledgements

I thank my supervisor Prof. David Jordan for his invaluable support and guidance throughout the project,

I would also like to thank Christopher Butler for providing the $\LaTeX$ code so I could easily set up the blueprint.

I would also like to thank Prof. Kevin Buzzard for his support and patience.

Finally, I would like to thank the many members of the Lean Zulip community who have provided insightful ideas and comments that have helped me progress much faster than otherwise.

Contributing

Contributions are welcome! If you would like to contribute, I recommend looking through the reference below and contacting me via zulip so I can find you a suitable task. At the time of writing, I am formalising lemma 2.3 iv) b); and hope to be soon formalising the inequality on page 35, which will lead to the case split on the classification of the arbitrary finite group.

Source reference

CB : ChristopherButlerExpositionOfDicksonsClassificationTheorem

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