Abstract
The objective of this chapter is to introduce the concepts of Lie groups and their Lie algebras. The Lie algebra \(\mathfrak {g}\) of a Lie group G is defined as the space of invariant vector fields (left or right, depending on choice), with bracket given by the Lie bracket of vector fields. The flows of invariant vector fields establish the exponential map \(\exp :\mathfrak {g}\rightarrow G\), which is the main link between \(\mathfrak {g}\) and G. These constructions make exhaustive use of results about vector fields on manifolds and their Lie brackets. A short collection of such results can be found in Appendix A. Adjoint representations are another tool for linking Lie groups and their Lie algebras. The formulas involving such representations are developed in this chapter. They are used along the whole theory.
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Notes
- 1.
Hilbert’s fifth problem (of 23 problems publicized in 1900) asks which topological groups are differentiable. A solution to this problem shows that a topological group is a Lie group if it is a (locally Euclidean) topological manifold. More generally, a locally compact group is a Lie group if it does not admit “small subgroups” (any neighborhood of the identity contains only the trivial subgroup). See Montgomery and Zippin [40] and Yang [60].
- 2.
In this statement, X denotes both an invariant vector field and an element of T 1 G. This apparent ambiguity arises from the isomorphism between the spaces of invariant vector fields and the tangent space T 1 G.
References
MONTGOMERY, D. and ZIPPIN, L. Topological transformation groups. Interscience, 1955.
YANG, C. T. “Hilbert’s fifth problem and related problems on transformation groups”. Mathematical developments arising from Hilbert problems. American Mathematical Society, Providence R. I., 1976, pp. 142–146 (Proceedings Symposia in Pure Math., vol. 28).
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San Martin, L.A.B. (2021). Lie Groups and Lie Algebras. In: Lie Groups. Latin American Mathematics Series. Springer, Cham. https://doi.org/10.1007/978-3-030-61824-7_5
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DOI: https://doi.org/10.1007/978-3-030-61824-7_5
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