Lie groups #
A Lie group is a group that is also a C^n manifold, in which the group operations of
multiplication and inversion are C^n maps. Regularity of the group multiplication means that
multiplication is a C^n mapping of the product manifold G × G into G.
Note that, since a manifold here is not second-countable and Hausdorff a Lie group here is not guaranteed to be second-countable (even though it can be proved it is Hausdorff). Note also that Lie groups here are not necessarily finite dimensional.
Main definitions #
LieAddGroup I G: a Lie additive group whereGis a manifold on the model with cornersI.LieGroup I G: a Lie multiplicative group whereGis a manifold on the model with cornersI.ContMDiffInv₀: typeclass forC^nmanifolds with0andInvsuch that inversion isC^nmap at each non-zero point. This includes complete normed fields and (multiplicative) Lie groups.
Main results #
ContMDiff.inv,ContMDiff.divand variants: point-wise inversion and division of mapsM → GisC^n.ContMDiff.inv₀and variants: ifContMDiffInv₀ I n N, point-wise inversion ofC^nmapsf : M → NisC^nat all points at whichfdoesn't vanish.ContMDiff.div₀and variants: if alsoContMDiffMul I n N(i.e.,Nis a Lie group except possibly for smoothness of inversion at0), similar results hold for point-wise division.instNormedSpaceLieAddGroup: a normed vector space over a nontrivially normed field is an additive Lie group.Instances/UnitsOfNormedAlgebrashows that the group of units of a complete normed𝕜-algebra is a multiplicative Lie group.
Implementation notes #
A priori, a Lie group here is a manifold with corners.
The definition of Lie group cannot require I : ModelWithCorners 𝕜 E E with the same space as the
model space and as the model vector space, as one might hope, because in the product situation,
the model space is ModelProd E E' and the model vector space is E × E', which are not the same,
so the definition does not apply. Hence the definition should be more general, allowing
I : ModelWithCorners 𝕜 E H.
An additive Lie group is a group and a C^n manifold at the same time in which
the addition and negation operations are C^n.
- compatible {e e' : PartialHomeomorph G H} : e ∈ atlas H G → e' ∈ atlas H G → e.symm.trans e' ∈ contDiffGroupoid n I
- contMDiff_add : ContMDiff (I.prod I) I n fun (p : G × G) => p.1 + p.2
Negation is smooth in an additive Lie group.
Instances
A (multiplicative) Lie group is a group and a C^n manifold at the same time in which
the multiplication and inverse operations are C^n.
- compatible {e e' : PartialHomeomorph G H} : e ∈ atlas H G → e' ∈ atlas H G → e.symm.trans e' ∈ contDiffGroupoid n I
- contMDiff_mul : ContMDiff (I.prod I) I n fun (p : G × G) => p.1 * p.2
Inversion is smooth in a Lie group.
Instances
Smoothness of inversion, negation, division and subtraction #
Let f : M → G be a C^n function into a Lie group, then f is point-wise
invertible with smooth inverse f. If f and g are two such functions, the quotient
f / g (i.e., the point-wise product of f and the point-wise inverse of g) is also C^n.
In a Lie group, inversion is C^n.
In an additive Lie group, inversion is a smooth map.
A Lie group is a topological group. This is not an instance for technical reasons, see note [Design choices about smooth algebraic structures].
An additive Lie group is an additive topological group. This is not an instance for technical reasons, see note [Design choices about smooth algebraic structures].
Binary product of Lie groups
Normed spaces are Lie groups #
C^n manifolds with C^n inversion away from zero #
Typeclass for C^n manifolds with 0 and Inv such that inversion is C^n at all non-zero
points. (This includes multiplicative Lie groups, but also complete normed semifields.)
Point-wise inversion is C^n when the function/denominator is non-zero.
A C^n manifold with 0 and Inv such that fun x ↦ x⁻¹ is C^n at all nonzero points.
Any complete normed (semi)field has this property.
- contMDiffAt_inv₀ ⦃x : G⦄ : x ≠ 0 → ContMDiffAt I I n (fun (y : G) => y⁻¹) x
Inversion is
C^naway from0.
Instances
In a manifold with C^n inverse away from 0, the inverse is continuous away from 0.
This is not an instance for technical reasons, see
note [Design choices about smooth algebraic structures].
Point-wise division of C^n functions #
If [ContMDiffMul I n N] and [ContMDiffInv₀ I n N], point-wise division of C^n
functions f : M → N is C^n whenever the denominator is non-zero.
(This includes N being a completely normed field.)