Does the Lie Bracket Always Close the Parallelogram as Described?

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SUMMARY

The discussion centers on the behavior of the Lie bracket in the context of smooth manifolds, specifically regarding its role in closing a "parallelogram" formed by the flows of vector fields. The example utilizes vector fields defined on ##\mathbb{RP}^3##, with calculations showing that the composition of flows leads to a closure of the parallelogram as described by Spivak in his work, "A Comprehensive Introduction to Differential Geometry, Volume 1." The key conclusion is that the flow of the Lie bracket indeed serves to complete this geometric structure, confirming its utility in differential geometry.

PREREQUISITES
  • Understanding of smooth manifolds and their structures.
  • Familiarity with vector fields and their flows.
  • Knowledge of the Lie derivative and Lie bracket concepts.
  • Basic comprehension of Riemannian geometry as presented in Manfredo do Carmo's texts.
NEXT STEPS
  • Study the properties of the Lie derivative in detail.
  • Explore the implications of the Lie bracket in various geometric contexts.
  • Learn about the applications of flows in differential geometry.
  • Investigate further examples in Spivak's "A Comprehensive Introduction to Differential Geometry."
USEFUL FOR

Mathematicians, particularly those specializing in differential geometry, researchers working with smooth manifolds, and students seeking to deepen their understanding of vector fields and their applications in geometric contexts.

Pond Dragon
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I finally finished my big summer research project. Reviewing how it went, it is clear to me that I lack an understanding of many structures on smooth manifolds. I decided to pull out my old copy of Spivak's A Comprehensive Introduction to Differential Geometry, Volume 1.

I'm currently working through the concept of the Lie derivative, which I don't actually remember from before. I decided that, to build understanding, I'd do a simple example on ##\mathbb{RP}^3##.

Fix a chart ##x:\{(p^1,p^2,p^3,p^4)\in\mathbb{R}^4~\vert~p^4\neq 0\}/\sim\,\to\mathbb{R}^3,~[p^1,p^2,p^3,p^4]\mapsto (\frac{p^1}{p^4},\frac{p^2}{p^4},\frac{p^3}{p^4})##, where ##\sim## is an equivalence relation defined by ##v \sim \lambda v##, for ##v\in\mathbb{R}^4## and ##\lambda\in\mathbb{R}##. This notation is used in Manfredo do Carmo's Riemannian Geometry. Consider vector fields ##X=\frac{\partial}{\partial x^1}## and ##Y=x^2\frac{\partial}{\partial x^1}+x^1\frac{\partial}{\partial x^3}##.

I calculated ##[X,Y]=\frac{\partial}{\partial x^3}##, and calculated the flows as $$\phi^X_t(p)=x^{-1}(t+x^1(p),x^2(p),x^3(p)) \\ \phi^Y_t(p)=x^{-1}(tx^2(p)+x^1(p),x^2(p),tx^1(p)+x^3(p)).$$

This makes the composition ##\phi^Y_{-t}\circ\phi^X_{-t}\circ\phi^Y_t\circ\phi^X_t(p)=x^{-1}(x^1(p),x^2(p),t^2(1-x^2(p))+x^3(p))##. The flow ##\phi^{[X,Y]}_t(p)=x^{-1}(x^1(p),x^2(p),t+x^3(p))## completes this "parallelogram" thing that Spivak talks about by the composition $$\phi^{[X,Y]}_{-t^2(1-x^2(p))}\circ\phi^Y_{-t}\circ\phi^X_{-t}\circ\phi^Y_t\circ\phi^X_t(p)=p.$$

Questions: Does this always happen? Can we then think of the flow of the Lie bracket as something that will close this "parallelogram"? If so, in what way?

Thank you!
 
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I made a small mistake.

$$\phi^Y_{-t}\circ\phi^X_{-t}\circ\phi^Y_t\circ\phi^X_t(p)=x^{-1}(x^1(p),x^2(p),t^2+x^3(p)).$$

I apologize.
 

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