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fixing stuff in groups.qmd
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mateuszbaran committed Aug 15, 2024
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Expand Up @@ -153,8 +153,9 @@ for aV in [aV1; aV2; aV3]
end
```

!!! note
In general, actions are usually non-commutative and the user must therefore be weary of [`LeftAction`](@ref) or [`RightAction`](@ref) needs. As in this case, the default `LeftAction()` is used.
::: {.callout-note}
In general, actions are usually non-commutative and the user must therefore be weary of [`LeftAction`](@ref) or [`RightAction`](@ref) needs. As in this case, the default `LeftAction()` is used.
:::

Finally, the actions (i.e. points from a manifold) can be [`compose`](@ref)d together. Consider putting together two rotations `aRb` and `bRc` such that a single composite rotation `aRc` is found. The next bit of code composes five rotations of `π/4` increments:
```{julia}
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## Relationship between groups, metrics and connections

Group structure provides a canonical way to define [📖 exponential](https://en.wikipedia.org/wiki/Exponential_map_(Lie_theory)) and 📖 logarithmic maps from the Lie algebra.
Group structure provides a canonical way to define [📖 exponential](https://en.wikipedia.org/wiki/Exponential_map_(Lie_theory)) and logarithmic maps from the Lie algebra.
They can be calculated in `Manifolds.jl` using the [`exp_lie`](@ref) and [`log_lie`](@ref) functions. Such exponential and logarithmic maps can be extended invariantly to tangent spaces at any point of the Lie group. This extension is implemented using functions [`exp_inv`](@ref) and [`log_inv`](@ref).

Finally, there are `log` and `exp` functions which are metric (or connection)-related functions in `Manifolds.jl`. For groups which can be equipped with a bi-invariant metric, `log` and `log_inv` return the same result, similarly `exp` and `exp_inv`. However, only compact groups and their products with Euclidean spaces can have a bi-invariant metric (see for example Theorem 21.9 in [GallierQuaintance:2020](@cite)). A prominent example of a Lie group without a bi-invariant metric is the special Euclidean group (in two or more dimensions). Then we have a choice between a metric but non-invariant exponential map (which is generally the default choice for `exp`) or a non-metric, invariant exponential map (`exp_inv`). Which one should be used depends on whether being metric or being invariant is more important in a particular application.
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The following table outlines invariance of `exp` and `log` of various groups.

| Group | Zero torsion connection | Invariant |
| --- | --- | --- | --- |
| --- | --- | --- |
| `ProductGroup` | Product of connections in each submanifold | Yes if both connections are invariant separately, otherwise no |
| `SemidirectProductGroup` | Same as underlying product ||
| `TranslationGroup` | `CartanSchoutenZero` ||
| `CircleGroup` | `CartanSchoutenZero` ||
| `GeneralLinearGroup` | Metric connection from the left invariant metric induced from the standard basis on the Lie algebra ||
| `GeneralUnitaryMultiplicationGroup` | `CartanSchoutenZero` (explicitly as such in `GeneralUnitaryMatrices.jl:225`) ||
| `HeisenbergGroup` | `CartanSchoutenZero`? (probably defined explicitly as such in `heisenberg.jl:220`) ||
| `GeneralUnitaryMultiplicationGroup` | `CartanSchoutenZero` (explicitly) ||
| `HeisenbergGroup` | `CartanSchoutenZero` ||
| `SpecialLinearGroup` | Same as `GeneralLinear` ||

: Invariance of `exp` and `log` for various groups {#tbl-exp-log-inv}
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