Properties of Maximal Tori

Oct 13, 2025 by wbrum.org 144 views

Introduction

In the realm of Lie groups and Lie algebras, two fundamental concepts play a crucial role in understanding the structure and properties of these mathematical objects. Maximal tori and Cartan subalgebras are essential components in the study of Lie groups, and their interplay is vital in determining the nature of these groups. In this article, we will delve into the world of maximal tori and Cartan subalgebras, exploring their definitions, properties, and significance in the context of Lie groups and Lie algebras.

Maximal Tori

A maximal torus in a compact Lie group $G$ is a maximal abelian subgroup $T$ that is also a torus, meaning it is isomorphic to a product of circles. In other words, $T$ is a closed, connected, and abelian subgroup of $G$ that is not contained in any larger abelian subgroup. The existence of maximal tori is a fundamental property of compact Lie groups, and they play a crucial role in the study of these groups.

Properties of Maximal Tori

Maximality: A maximal torus is a maximal abelian subgroup, meaning it is not contained in any larger abelian subgroup.
Closedness: A maximal torus is a closed subgroup, meaning it is a closed subset of the Lie group $G$ .
Connectedness: A maximal torus is a connected subgroup, meaning it is connected as a topological space.
Abelianess: A maximal torus is an abelian subgroup, meaning its elements commute with each other.

Cartan Subalgebras

A Cartan subalgebra of a Lie algebra $\mathfrak{g}$ is a subalgebra $\mathfrak{h}$ that is nilpotent, self-normalizing, and contains the center of $\mathfrak{g}$ . In other words, $\mathfrak{h}$ is a subalgebra of $\mathfrak{g}$ that satisfies the following properties:

Nilpotency: $\mathfrak{h}$ is a nilpotent Lie algebra, meaning that its lower central series eventually terminates.
Self-normalizing: $\mathfrak{h}$ is self-normalizing, meaning that the normalizer of $\mathfrak{h}$ in $\mathfrak{g}$ is equal to $\mathfrak{h}$ itself.
Center: $\mathfrak{h}$ contains the center of $\mathfrak{g}$ , meaning that it contains all elements of $\mathfrak{g}$ that commute with every element of $\mathfrak{g}$ .

Properties of Cartan Subalgebras

Nilpotency: A Cartan subalgebra is a nilpotent Lie algebra, meaning that its lower central series eventually terminates.
Self-normalizing: A Cartan subalgebra is self-normalizing, meaning that the normalizer of the Cartan subalgebra in the Lie algebra is equal to the Cartan subalgebra itself.
Center: A Cartan subalgebra contains the center of the Lie algebra, meaning that it contains all elements of the Lie algebra that commute with every element of the Lie algebra.

Relationship between Maximal Tori and Cartan Subalgebras

The relationship between maximal tori and Cartan subalgebras is a fundamental aspect of the study of Lie groups and Lie algebras. In fact, the existence of a maximal torus in a compact Lie group $G$ is equivalent to the existence of a Cartan subalgebra in its Lie algebra $\mathfrak{g}$ . This relationship is a consequence of the following theorem:

Theorem: Let $G$ be a compact Lie group, and let $\mathfrak{g}$ be its Lie algebra. Then, the following statements are equivalent:

$G$ has a maximal torus $T$ .
$\mathfrak{g}$ has a Cartan subalgebra $\mathfrak{h}$ .

Proof: The proof of this theorem is a consequence of the following steps:

Step 1: Let $T$ be a maximal torus in $G$ . Then, the Lie algebra $\mathfrak{t}$ of $T$ is a Cartan subalgebra of $\mathfrak{g}$ .
Step 2: Let $\mathfrak{h}$ be a Cartan subalgebra of $\mathfrak{g}$ . Then, the exponential map $\exp: \mathfrak{h} \to G$ is a diffeomorphism, and its image is a maximal torus in $G$ .

Conclusion

Q: What is a maximal torus in a compact Lie group?

A: A maximal torus in a compact Lie group $G$ is a maximal abelian subgroup $T$ that is also a torus, meaning it is isomorphic to a product of circles. In other words, $T$ is a closed, connected, and abelian subgroup of $G$ that is not contained in any larger abelian subgroup.

Q: What are the properties of a maximal torus?

A: A maximal torus has the following properties:

Maximality: A maximal torus is a maximal abelian subgroup, meaning it is not contained in any larger abelian subgroup.
Closedness: A maximal torus is a closed subgroup, meaning it is a closed subset of the Lie group $G$ .
Connectedness: A maximal torus is a connected subgroup, meaning it is connected as a topological space.
Abelianess: A maximal torus is an abelian subgroup, meaning its elements commute with each other.

Q: What is a Cartan subalgebra of a Lie algebra?

A: A Cartan subalgebra of a Lie algebra $\mathfrak{g}$ is a subalgebra $\mathfrak{h}$ that is nilpotent, self-normalizing, and contains the center of $\mathfrak{g}$ . In other words, $\mathfrak{h}$ is a subalgebra of $\mathfrak{g}$ that satisfies the following properties:

Nilpotency: $\mathfrak{h}$ is a nilpotent Lie algebra, meaning that its lower central series eventually terminates.
Self-normalizing: $\mathfrak{h}$ is self-normalizing, meaning that the normalizer of $\mathfrak{h}$ in $\mathfrak{g}$ is equal to $\mathfrak{h}$ itself.
Center: $\mathfrak{h}$ contains the center of $\mathfrak{g}$ , meaning that it contains all elements of $\mathfrak{g}$ that commute with every element of $\mathfrak{g}$ .

Q: What are the properties of a Cartan subalgebra?

A: A Cartan subalgebra has the following properties:

Nilpotency: A Cartan subalgebra is a nilpotent Lie algebra, meaning that its lower central series eventually terminates.
Self-normalizing: A Cartan subalgebra is self-normalizing, meaning that the normalizer of the Cartan subalgebra in the Lie algebra is equal to the Cartan subalgebra itself.
Center: A Cartan subalgebra contains the center of the Lie algebra, meaning that it contains all elements of the Lie algebra that commute with every element of the Lie algebra.

Q: What is the relationship between maximal tori and Cartan subalgebras?

A: The existence of a maximal torus in a compact Lie group $G$ is equivalent to the existence of a Cartan subalgebra in its Lie algebra $\mathfrak{g}$ . This relationship is a consequence of the theorem stated above.

Q: How do I find a maximal torus in a compact Lie group?

A: To find a maximal torus in a compact Lie group $G$ , you can use the following steps:

Step 1: Find a maximal abelian subgroup $T$ of $G$ .
Step 2: Check if $T$ is a torus, meaning it is isomorphic to a product of circles.
Step 3: If $T$ is a torus, then it is a maximal torus in $G$ .

Q: How do I find a Cartan subalgebra in a Lie algebra?

A: To find a Cartan subalgebra in a Lie algebra $\mathfrak{g}$ , you can use the following steps:

Step 1: Find a nilpotent subalgebra $\mathfrak{h}$ of $\mathfrak{g}$ .
Step 2: Check if $\mathfrak{h}$ is self-normalizing, meaning that the normalizer of $\mathfrak{h}$ in $\mathfrak{g}$ is equal to $\mathfrak{h}$ itself.
Step 3: If $\mathfrak{h}$ is self-normalizing, then it is a Cartan subalgebra of $\mathfrak{g}$ .

Q: What are the applications of maximal tori and Cartan subalgebras?

A: Maximal tori and Cartan subalgebras have far-reaching implications for the study of compact Lie groups and Lie algebras. Some of the applications of these concepts include:

Representation theory: Maximal tori and Cartan subalgebras play a crucial role in the study of representations of compact Lie groups and Lie algebras.
Symplectic geometry: Maximal tori and Cartan subalgebras are used in the study of symplectic geometry, particularly in the context of symplectic Lie groups and Lie algebras.
Topology: Maximal tori and Cartan subalgebras are used in the study of topology, particularly in the context of topological invariants of compact Lie groups and Lie algebras.

Conclusion

In conclusion, maximal tori and Cartan subalgebras are fundamental concepts in the study of Lie groups and Lie algebras. The relationship between these two concepts is a crucial aspect of the study of compact Lie groups, and it has far-reaching implications for the study of these groups. By understanding the properties and applications of maximal tori and Cartan subalgebras, we can gain a deeper insight into the structure and behavior of compact Lie groups and Lie algebras.