Cartan subgroup

In the theory of algebraic groups, a Cartan subgroup of a connected linear algebraic group $G$ over a (not necessarily algebraically closed) field $k$ is the centralizer of a maximal torus. Cartan subgroups are smooth (equivalently reduced), connected and nilpotent. If $k$ is algebraically closed, they are all conjugate to each other.

Notice that in the context of algebraic groups a torus is an algebraic group $T$ such that the base extension $T_{(\overline{k})}$ (where $\overline{k}$ is the algebraic closure of $k$) is isomorphic to the product of a finite number of copies of the ${\mathbf{𝐆}}_m={\mathbf{𝐆}\mathbf{𝐋}}_1$. Maximal such subgroups have in the theory of algebraic groups a role that is similar to that of maximal tori in the theory of Lie groups.

If $G$ is reductive (in particular, if it is semi-simple), then a torus is maximal if and only if it is its own centraliser and thus Cartan subgroups of $G$ are precisely the maximal tori.

Example

The general linear groups ${\mathbf{𝐆}\mathbf{𝐋}}_n$ are reductive. The diagonal subgroup is clearly a torus (indeed a split torus, since it is product of n copies of ${\mathbf{𝐆}}_m$ already before any base extension), and it can be shown to be maximal. Since ${\mathbf{𝐆}\mathbf{𝐋}}_n$ is reductive, the diagonal subgroup is a Cartan subgroup.

See also

• Borel subgroup
• Algebraic group
• Algebraic torus

References

• Borel, Armand (1991-12-31). Linear algebraic groups. ISBN 3-540-97370-2. 
• Lang, Serge (2002). Algebra. Springer. ISBN 978-0-387-95385-4. 
• Algebraic Groups: The Theory of Group Schemes of Finite Type over a Field, Cambridge University Press, 2017, doi:10.1017/9781316711736, ISBN 978-1107167483 
• Hazewinkel, Michiel, ed. (2001), "Cartan subgroup", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=Main_Page 
• Springer, Tonny A. (1998), Linear algebraic groups, Progress in Mathematics, 9 (2nd ed.), Boston, MA: Birkhäuser Boston, ISBN 978-0-8176-4021-7