Talk:Lattice (discrete subgroup)

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The comment(s) below were originally left at Talk:Lattice (discrete subgroup)/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

Hard to assign a unique field… Arcfrk (talk) 10:46, 16 February 2008 (UTC)[reply]

Last edited at 10:46, 16 February 2008 (UTC). Substituted at 21:42, 29 April 2016 (UTC)

Seems a one-sided viewpoint

"Lattices are best thought of as discrete approximations of continuous groups (such as Lie groups)."

This isn't the best viewpoint if your interest is the quotient space.

188.154.206.128 (talk) 16:15, 21 January 2019 (UTC)[reply]

Quasi-isometry and coarse equivalence

I'm not sure whether discussion of these notions is relevant in the introductory section. As far as i can tell it is equivalent for a discrete subgroup to be either a uniform lattice, quasi-isometric to or coarsely equivalent to its ambient group (with an invariant metric), though i don't know any reference for the latter.

It could make sense to add a section about "lattices in geometric group theory" or something similar where this is discussed. jraimbau (talk) 12:11, 27 October 2021 (UTC)[reply]

Is "cocompact" a synonym of "uniform"?

In the section Generalities on lattices this sentence appears:

"A lattice $\Gamma \subset G$ is called uniform when the quotient space $G/\Gamma$ is compact (and non-uniform otherwise)."

Am I correct to say that, when speaking of a lattice in a Lie group, "cocompact" is a synonym for "uniform"?

If so, then this is worth mentioning in the artice. 2601:200:C000:1A0:C0A2:E29D:72EF:28D2 (talk) 19:10, 12 May 2022 (UTC)[reply]

yes, done. jraimbau (talk) 06:00, 13 May 2022 (UTC)[reply]

Unclear terminology

The section "Rank 1 versus higher rank begins with this sentence:

"The real rank of a Lie group is the maximal dimension of an abelian subgroup containing only semisimple elements.'

The linked article Semisimple does not explain what a "semisimple element" is.

I hope someone knowledgeable about this subject can clarify this. 2601:200:C000:1A0:C0A2:E29D:72EF:28D2 (talk) 19:20, 12 May 2022 (UTC)[reply]

The article on semisimple operators does define what it means pretty clearly (an element of a Lie group is a linear operator via a faithful linear representation of the Lie group, for instance the adjoint representation, and semisimplicity does not depend on the representation). jraimbau (talk) 05:06, 13 May 2022 (UTC)[reply]

Lattices of nilpotent Lie groups

In the section about lattices of nilpotent Lie groups, I am doubtful of the following statement (which also happens not to have any reference backing it up): “More precisely: In a nilpotent group whose Lie algebra has only rational structure constants, lattices are the images via the exponential map of lattices (in the more elementary sense of Lattice (group)) in the Lie algebra.”

Since the exponential map is not a morphism unless the group is abelian, yet is a diffeomorphism (in the case of nilpotent connected and simply connected Lie groups, by Varadarajan Lie groups, Lie algebras and their representations, Thm. 3.6.2), it indeed seems highly improbable that this is the case. A quick computation in the case of the Heisenberg group $H=\left\{{\begin{pmatrix}1&a&c\\0&1&b\\0&0&1\end{pmatrix}},a,b,c\in \mathbb {Z} \right\}$ shows that for all $a,b,c\in \mathbb {R}$ ,

$\exp {\begin{pmatrix}0&a&c\\0&0&b\\0&0&0\end{pmatrix}}={\begin{pmatrix}1&a&c+{\frac {1}{2}}ab\\0&1&b\\0&0&1\end{pmatrix}}$ lies in $H$ if and only if $a,b\in \mathbb {Z}$ and $c\in \mathbb {Z}$ if $ab$ is even, and $c\in \mathbb {Z} +{\frac {1}{2}}$ if $ab$ is odd.

This clearly shows that although both $(0,1,0)$ and $(1,0,0)$ lie in $\exp ^{-1}H$ , their sum $(1,1,0)$ does not. So $\exp ^{-1}H$ is not a lattice of the Lie algebra (in the abelian group sense).

Am I missing something trivial? In this case, perhaps adding a reference would not be superfluous anyway! 176.132.228.101 (talk) 20:44, 10 August 2024 (UTC)[reply]

You are right, there is a relation between lattices in the Lie group and the Lie algebra but the correct statement is a bit subtler than the erroneous one in the article. In Raghunathan's book just after Theorem 2.12 (the criterion on structure constants for the existence of lattices) it is stated that the arguments establish the more precise fact that if N, n are a connected, simply connected nilpotent Lie group and its Lie algebra, if L is a lattice in n then exp(L) generates a lattice in N, and if G is a lattice in N then exp^{-1}(G) generates a lattice in n. I'll amend the statement in the article accordingly. jraimbau (talk) 06:24, 11 August 2024 (UTC)[reply]