- #1
mnb96
- 715
- 5
Hello,
I have troubles formulating this question properly. So I will explain it through one example.
If we consider the Lie group R=SO(2) of rotations on the plane, we know that we can find a manifold on which the group SO(2) acts regularly: this manifold is the unit circle in ℝ2. In fact, there is a bijection between points of the circle and elements of SO(2), and furthermore by letting SO(2) act on one arbitrary point x of the circle, the point x "orbits" smoothly along the circle.
Similarly if we consider the Lie group T of translations on the plane, we know that T acts regularly on ℝ2. In fact there is a 1-to-1 mapping between elements of T and the points of ℝ2, and if we choose one point x in ℝ2, the action of T on x will make x orbit smoothly across the manifold ℝ2.
Now the question is: if I consider a group G=RT (i.e. the group of rotations followed by translations), is it possible to find some manifold M on which G acts regularly?
In the above case, G is clearly a Lie group of dimension 3, but it does not act regularly on the points of the manifold [itex]M=[0,2\pi)\times \mathbb{R}^2[/itex] because R and T are not commutative (T is only a normal subgroup of G), and there is no 1-to-1 correspondence between the points on the orbit of a point x[itex]\in[/itex]M and the elements of G.
I have troubles formulating this question properly. So I will explain it through one example.
If we consider the Lie group R=SO(2) of rotations on the plane, we know that we can find a manifold on which the group SO(2) acts regularly: this manifold is the unit circle in ℝ2. In fact, there is a bijection between points of the circle and elements of SO(2), and furthermore by letting SO(2) act on one arbitrary point x of the circle, the point x "orbits" smoothly along the circle.
Similarly if we consider the Lie group T of translations on the plane, we know that T acts regularly on ℝ2. In fact there is a 1-to-1 mapping between elements of T and the points of ℝ2, and if we choose one point x in ℝ2, the action of T on x will make x orbit smoothly across the manifold ℝ2.
Now the question is: if I consider a group G=RT (i.e. the group of rotations followed by translations), is it possible to find some manifold M on which G acts regularly?
In the above case, G is clearly a Lie group of dimension 3, but it does not act regularly on the points of the manifold [itex]M=[0,2\pi)\times \mathbb{R}^2[/itex] because R and T are not commutative (T is only a normal subgroup of G), and there is no 1-to-1 correspondence between the points on the orbit of a point x[itex]\in[/itex]M and the elements of G.