LetGbe a discrete subgroup ofIso(R2). Show that every subgroup ofGis also discrete.

Isn't this true simply because it's a subgroup? So the elements of the subgroup are also in G?

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- Nov 22nd 2011, 03:16 PMmonomocoDiscrete Isometries
Let

*G*be a discrete subgroup of*I**s**o*(R2). Show that every subgroup of*G*is also discrete.

Isn't this true simply because it's a subgroup? So the elements of the subgroup are also in G? - Nov 22nd 2011, 08:39 PMDrexel28Re: Discrete Isometries
I'm confused, if you are talking about the fact that a subgroup of a discrete subgroup is discrete, this really is just the (more general statement) that a subspace of a discrete space is discrete. In particular, you want to show that each element of is open, but for each you know (via the fact that is discrete) that there exists some open set such that . Clearly though and so is open in . Since was arbitrary the conclusion follows. Make sense?

- Nov 22nd 2011, 08:44 PMmonomocoRe: Discrete Isometries
We're talking about a subgroup of Iso (R2) so by discrete I have the definition:

G< Iso (R2) is discrete there exists an E such that

for all translation t_a in G, a < E

for all rotations in r_o in G, o<E

so the motions cannot be arbitrarily small. I understand what you mean, but I don't know how to apply it to this example. - Nov 22nd 2011, 08:49 PMDrexel28Re: Discrete Isometries
- Nov 22nd 2011, 08:52 PMmonomocoRe: Discrete Isometries
I thought I must be missing something. Thanks for your help!