Dempwolff group

In mathematical finite group theory, the Dempwolff group is a finite group of order 319979520 = 2^15·3^2·5·7·31, that is the unique nonsplit extension ${\displaystyle 2^{5\,.}\mathrm {GL} _{5}(\mathbb {F} _{2})}$ of ${\displaystyle \mathrm {GL} _{5}(\mathbb {F} _{2})}$ by its natural module of order ${\displaystyle 2^{5}}$. The uniqueness of such a nonsplit extension was shown by Dempwolff (1972), and the existence by Thompson (1976), who showed using some computer calculations of Smith (1976) that the Dempwolff group is contained in the compact Lie group ${\displaystyle E_{8}}$ as the subgroup fixing a certain lattice in the Lie algebra of ${\displaystyle E_{8}}$, and is also contained in the Thompson sporadic group (the full automorphism group of this lattice) as a maximal subgroup.

Huppert (1967, p.124) showed that any extension of ${\displaystyle \mathrm {GL} _{n}(\mathbb {F} _{q})}$ by its natural module ${\displaystyle \mathbb {F} _{q}^{n}}$ splits if ${\displaystyle q>2}$. Note that this theorem does not necessarily apply to extensions of ${\displaystyle \mathrm {SL} _{n}(\mathbb {F} _{q})}$; for example, there is a non-split extension ${\displaystyle 5^{3\,.}\mathrm {SL} _{n}(\mathbb {F} _{q})}$, which is a maximal subgroup of the Lyons group. Dempwolff (1973) showed that it also splits if ${\displaystyle n}$ is not 3, 4, or 5, and in each of these three cases there is just one non-split extension. These three nonsplit extensions can be constructed as follows:

• The nonsplit extension ${\displaystyle 2^{3\,.}\mathrm {GL} _{3}(\mathbb {F} _{2})}$ is a maximal subgroup of the Chevalley group ${\displaystyle G_{2}(\mathbb {F} _{3})}$.
• The nonsplit extension ${\displaystyle 2^{4\,.}\mathrm {GL} _{4}(\mathbb {F} _{2})}$ is a maximal subgroup of the sporadic Conway group Co₃.
• The nonsplit extension ${\displaystyle 2^{5\,.}\mathrm {GL} _{5}(\mathbb {F} _{2})}$ is a maximal subgroup of the Thompson sporadic group Th.

References

• Dempwolff, Ulrich (1972), "On extensions of an elementary abelian group of order 2^5 by GL(5,2)", Rendiconti del Seminario Matematico della Università di Padova. The Mathematical Journal of the University of Padova, 48: 359–364, ISSN 0041-8994, MR 0393276
• Dempwolff, Ulrich (1973), "On the second cohomology of GL(n,2)", Australian Mathematical Society. Journal. Series A. Pure Mathematics and Statistics, 16: 207–209, doi:10.1017/S1446788700014221, ISSN 0263-6115, MR 0357639
• Griess, Robert L. (1976), "On a subgroup of order 2^15 . ¦GL(5,2)¦ in E₈(C), the Dempwolff group and Aut(D₈°D₈°D₈)" (PDF), Journal of Algebra, 40 (1): 271–279, doi:10.1016/0021-8693(76)90097-1, hdl:2027.42/21778, ISSN 0021-8693, MR 0407149
• Huppert, Bertram (1967), Endliche Gruppen (in German), Berlin, New York: Springer-Verlag, ISBN 978-3-540-03825-2, MR 0224703, OCLC 527050
• Smith, P. E. (1976), "A simple subgroup of M? and E₈(3)", The Bulletin of the London Mathematical Society, 8 (2): 161–165, doi:10.1112/blms/8.2.161, ISSN 0024-6093, MR 0409630
• Thompson, John G. (1976), "A conjugacy theorem for E₈", Journal of Algebra, 38 (2): 525–530, doi:10.1016/0021-8693(76)90235-0, ISSN 0021-8693, MR 0399193

External links

• Dempwolff group at the atlas of groups.