# Sh:1106

• Paolini, G., & Shelah, S. (2018). The automorphism group of Hall’s universal group. Proc. Amer. Math. Soc., 146(4), 1439–1445.
• Abstract:
We study the automorphism group of Hall’s universal locally finite group H. We show that in Aut(H) every subgroup of index < 2^\omega lies between the pointwise and the setwise stabilizer of a unique finite subgroup A of H, and use this to prove that Aut(H) is complete. We further show that Inn(H) is the largest locally finite normal subgroup of Aut(H). Finally, we observe that from the work of [312] it follows that for every countable locally finite G there exists G \cong G' \leq H such that every f \in Aut(G') extends to an \hat{f} \in Aut(H) in such a way that f \mapsto \hat{f} embeds Aut(G') into Aut(H). In particular, we solve the three open questions of Hickin on Aut(H) from [3] and give a partial answer to Question VI.5 of Kegel and Wehrfritz from [6].
• published version (7p)
Bib entry
@article{Sh:1106,
author = {Paolini, Gianluca and Shelah, Saharon},
title = {{The automorphism group of Hall's universal group}},
journal = {Proc. Amer. Math. Soc.},
fjournal = {Proceedings of the American Mathematical Society},
volume = {146},
number = {4},
year = {2018},
pages = {1439--1445},
issn = {0002-9939},
mrnumber = {3754331},
mrclass = {20B27 (03C60 20F50)},
doi = {10.1090/proc/13836},
note = {\href{https://arxiv.org/abs/1703.10540}{arXiv: 1703.10540}},
arxiv_number = {1703.10540}
}