Lie groups #
A Lie group is a group that is also a smooth manifold, in which the group operations of
multiplication and inversion are smooth maps. Smoothness of the group multiplication means that
multiplication is a smooth 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 whereG
is a manifold on the model with cornersI
.LieGroup I G
: a Lie multiplicative group whereG
is a manifold on the model with cornersI
.SmoothInv₀
: typeclass for smooth manifolds with0
andInv
such that inversion is a smooth map at each non-zero point. This includes complete normed fields and (multiplicative) Lie groups.
Main results #
ContMDiff.inv
,ContMDiff.div
and variants: point-wise inversion and division of mapsM → G
is smoothContMDiff.inv₀
and variants: ifSmoothInv₀ N
, point-wise inversion of smooth mapsf : M → N
is smooth at all points at whichf
doesn't vanish.ContMDiff.div₀
and variants: if alsoSmoothMul N
(i.e.,N
is a Lie group except possibly for smoothness of inversion at0
), similar results hold for point-wise division.normedSpaceLieAddGroup
: a normed vector space over a nontrivially normed field is an additive Lie group.Instances/UnitsOfNormedAlgebra
shows 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 smooth manifold at the same time in which the addition and negation operations are smooth.
Instances
Negation is smooth in an additive Lie group.
A (multiplicative) Lie group is a group and a smooth manifold at the same time in which the multiplication and inverse operations are smooth.
Instances
Inversion is smooth in a Lie group.
Smoothness of inversion, negation, division and subtraction #
Let f : M → G
be a C^n
or smooth functions 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 smooth.
In a Lie group, inversion is a smooth map.
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
Equations
- ⋯ = ⋯
Equations
- ⋯ = ⋯
Normed spaces are Lie groups #
Equations
- ⋯ = ⋯
Smooth manifolds with smooth inversion away from zero #
Typeclass for smooth manifolds with 0
and Inv
such that inversion is smooth at all non-zero
points. (This includes multiplicative Lie groups, but also complete normed semifields.)
Point-wise inversion is smooth when the function/denominator is non-zero.
A smooth manifold with 0
and Inv
such that fun x ↦ x⁻¹
is smooth at all nonzero points.
Any complete normed (semi)field has this property.
Inversion is smooth away from
0
.
Instances
Inversion is smooth away from 0
.
Equations
- ⋯ = ⋯
In a manifold with smooth 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 smooth functions #
If [SmoothMul I N]
and [SmoothInv₀ I N]
, point-wise division of smooth functions f : M → N
is smooth whenever the denominator is non-zero. (This includes N
being a completely normed field.)