## Abstract

This paper contributes to the generalization of Rademacher’s differentiability result for Lipschitz functions when the domain is infinite dimensional and has nonabelian group structure. We introduce an infinite-dimensional analogue of Carnot groups that are metric groups equipped with dilations (which we call metric scalable groups) admitting a dense increasing sequence of finite-dimensional Carnot subgroups. For such groups, we show that every Lipschitz function has a point of Gâteaux differentiability. As a step in the proof, we show that a certain \(\sigma \)-ideal of sets that are null with respect to this sequence of subgroups cannot contain open sets. We also give a geometric criterion for when such Carnot subgroups exist in metric scalable groups and provide examples of such groups. The proof of the main theorem follows the work of Aronszajn (Stud Math 57(2):147–190, 1976) and Pansu (Ann Math 129(1):1–60, 1989).

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E. L. D. and T. M. were partially supported by the Academy of Finland (Grant 288501 ‘*Geometry of subRiemannian groups*’) and by the European Research Council (ERC Starting Grant 713998 GeoMeG ‘*Geometry of Metric Groups*’). T. M. was also supported by the Finnish Cultural Foundation (Grant 00170709). S. L. was supported by NSF Grant DMS-1812879.

## Appendix A: Some Useful Commutator Identities

### Appendix A: Some Useful Commutator Identities

### Lemma A.1

Let *G* be a group and \( x,y,z\in G \). Then

where *h* is a product of commutators of *x*, *y*, *z* of weight \( \ge 3 \).

### Proof

For the first equation,

Since \( [a,b]={[b,a]}^{-1} \),

Using this and the identity \( [a,b]={[b,a]}^{-1} \),

The last equation follows by reordering the terms, which produces some higher order commutators into *h*. \(\square \)

### Corollary A.2

If \( [y,z]\in Z(G) \), then

### Corollary A.3

Let \( n,m\in {\mathbb {N}}\). Then

where *h* is a product of commutators of *x* and *y* of weight \( \ge 3 \).

### Proof

The proof is by iterating Lemma A.1 for *nm* times and reordering the terms, which produces some additional higher order commutators into *h*. \(\square \)

### Lemma A.4

Let *G* be a group, \( x,y\in G \). Then

### Proof

The statement follows from

\(\square \)

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Le Donne, E., Li, S. & Moisala, T. Infinite-Dimensional Carnot Groups and Gâteaux Differentiability.
*J Geom Anal* **31, **1756–1785 (2021). https://doi.org/10.1007/s12220-019-00324-x

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### Keywords

- Carnot groups
- Differentiability
- Rademacher
- Gateaux derivative

### Mathematics Subject Classification

- 28A15
- 53C17
- 58C20
- 46G05