# Difference between revisions of "Adjoint group"

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2010 Mathematics Subject Classification: Primary: 20GXX Secondary: 14LXX [MSN][ZBL]

The adjoint group of a linear group $G$ is the linear group $\def\Ad{\textrm{Ad}\;} \Ad G$ which is the image of the Lie group or algebraic group $G$ under the adjoint representation (cf. Adjoint representation of a Lie group). The adjoint group $\Ad G$ is contained in the group $\def\Aut{\textrm{Aut}\;} \def\g{\mathfrak g} \Aut \g$ of automorphisms of the Lie algebra $\g$ of $G$, and its Lie algebra coincides with the adjoint algebra $\Ad\g$ of $\g$. A connected semi-simple group is a group of adjoint type (i.e. is isomorphic to its adjoint group) if and only if its roots generate the lattice of rational characters of the maximal torus; the centre of such a group is trivial. If the ground field has characteristic zero and $G$ is connected, then $\Ad G$ is uniquely determined by the Lie algebra $\g$ and is either called the adjoint group or the group of inner automorphisms of $\g$. In particular, if $G$ is semi-simple, $\Ad G$ coincides with the connected component of the identity in $\Aut \g$.

#### References

 [Bo] N. Bourbaki, "Elements of mathematics. Lie groups and Lie algebras", Addison-Wesley (1975) pp. Chapt. 2; 3 (Translated from French) MR0682756 Zbl 0319.17002 [Hu] J.E. Humphreys, "Linear algebraic groups", Springer (1975) MR0396773 Zbl 0325.20039 [Po] L.S. Pontryagin, "Topological groups", Princeton Univ. Press (1958) (Translated from Russian) MR0201557 Zbl 0022.17104 [Se] J.-P. Serre, "Lie algebras and Lie groups", Benjamin (1965) (Translated from French) MR0218496 Zbl 0132.27803
How to Cite This Entry:
Adjoint group. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Adjoint_group&oldid=23561
This article was adapted from an original article by A.L. Onishchik (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article