Operator norm on the space of continuous alternating maps #
In this file we show that continuous alternating maps from a seminormed space to a (semi)normed space form a (semi)normed space. We also prove basic facts about this norm and define bundled versions of some operations on continuous alternating maps.
Most proofs just invoke the corresponding fact about continuous multilinear maps.
Type variables #
We use the following type variables in this file:
𝕜
: a nontrivially normed field;ι
: a finite index type;E
,F
,G
: (semi)normed vector spaces over𝕜
.
Applying a continuous alternating map to a vector is continuous in the pair (map, vector).
Continuity in in the vector holds by definition and continuity in the map holds if both the domain and the codomain are topological vector spaces. However, continuity in the pair (map, vector) needs the domain to be a locally bounded TVS. We have no typeclass for a locally bounded TVS, so we require it to be a seminormed space instead.
Continuity properties of alternating maps #
We relate continuity of alternating maps to the inequality ‖f m‖ ≤ C * ∏ i, ‖m i‖
, in
both directions. Along the way, we prove useful bounds on the difference ‖f m₁ - f m₂‖
.
If f
is a continuous alternating map on E
and m
is an element of ι → E
such that one of the m i
has norm 0
, then f m
has norm 0
.
Note that we cannot drop the continuity assumption.
Indeed, let ℝ₀
be a copy or ℝ
with zero norm and indiscrete topology.
Then f : (Unit → ℝ₀) → ℝ
given by f x = x ()
sends vector 1
with zero norm to number 1
with nonzero norm.
If an alternating map in finitely many variables on seminormed spaces
sends vectors with a component of norm zero to vectors of norm zero
and satisfies the inequality ‖f m‖ ≤ C * ∏ i, ‖m i‖
on a shell ε i / ‖c i‖ < ‖m i‖ < ε i
for some positive numbers ε i
and elements c i : 𝕜
, 1 < ‖c i‖
,
then it satisfies this inequality for all m
.
The first assumption is automatically satisfied on normed spaces, see bound_of_shell
below.
For seminormed spaces, it follows from continuity of f
,
see lemma bound_of_shell_of_continuous
below.
If a continuous alternating map in finitely many variables on normed spaces
satisfies the inequality ‖f m‖ ≤ C * ∏ i, ‖m i‖
on a shell ε / ‖c‖ < ‖m i‖ < ε
for some positive number ε
and an elements c : 𝕜
, 1 < ‖c‖
,
then it satisfies this inequality for all m
.
If the domain is a Hausdorff space, then the continuity assumption is reduntant,
see bound_of_shell
below.
If an alternating map in finitely many variables on a seminormed space is continuous,
then it satisfies the inequality ‖f m‖ ≤ C * ∏ i, ‖m i‖
,
for some C
which can be chosen to be positive.
If an alternating map f
satisfies a boundedness property around 0
,
one can deduce a bound on f m₁ - f m₂
using the multilinearity.
Here, we give a precise but hard to use version.
See AlternatingMap.norm_image_sub_le_of_bound
for a less precise but more usable version.
The bound reads
‖f m - f m'‖ ≤ C * ‖m 1 - m' 1‖ * max ‖m 2‖ ‖m' 2‖ * max ‖m 3‖ ‖m' 3‖ * ... * max ‖m n‖ ‖m' n‖ + ...
,
where the other terms in the sum are the same products where 1
is replaced by any i
.
If an alternating map f
satisfies a boundedness property around 0
,
one can deduce a bound on f m₁ - f m₂
using the multilinearity.
Here, we give a usable but not very precise version.
See AlternatingMap.norm_image_sub_le_of_bound'
for a more precise but less usable version.
The bound is ‖f m - f m'‖ ≤ C * card ι * ‖m - m'‖ * (max ‖m‖ ‖m'‖) ^ (card ι - 1)
.
If an alternating map satisfies an inequality ‖f m‖ ≤ C * ∏ i, ‖m i‖
,
then it is continuous.
Construct a continuous alternating map from a alternating map satisfying a boundedness condition.
Equations
- f.mkContinuous C H = { toMultilinearMap := ↑f, cont := ⋯, map_eq_zero_of_eq' := ⋯ }
Instances For
Continuous alternating maps #
We define the norm ‖f‖
of a continuous alternating map f
in finitely many variables
as the smallest nonnegative number such that ‖f m‖ ≤ ‖f‖ * ∏ i, ‖m i‖
for all m
.
We show that this defines a normed space structure on E [⋀^ι]→L[𝕜] F
.
Continuous alternating maps form a seminormed additive commutative group.
We override projection to PseudoMetricSpace
to ensure that instances commute
in with_reducible_and_instances
.
Equations
- One or more equations did not get rendered due to their size.
The inclusion of E [⋀^ι]→L[𝕜] F
into ContinuousMultilinearMap 𝕜 (fun _ : ι ↦ E) F
as a linear isometry.
Equations
- ContinuousAlternatingMap.toContinuousMultilinearMapLI = { toLinearMap := ContinuousAlternatingMap.toContinuousMultilinearMapLinear, norm_map' := ⋯ }
Instances For
The fundamental property of the operator norm of a continuous alternating map:
‖f m‖
is bounded by ‖f‖
times the product of the ‖m i‖
.
The image of the unit ball under a continuous alternating map is bounded.
If one controls the norm of every f x
, then one controls the norm of f
.
The fundamental property of the operator norm of a continuous alternating map:
‖f m‖
is bounded by ‖f‖
times the product of the ‖m i‖
, nnnorm
version.
Equations
- ContinuousAlternatingMap.instNormedSpace = { toModule := ContinuousAlternatingMap.instModule, norm_smul_le := ⋯ }
ContinuousAlternatingMap.ofSubsingleton
as a linear isometry.
Equations
- One or more equations did not get rendered due to their size.
Instances For
ContinuousAlternatingMap.prod
as a LinearIsometryEquiv
.
Equations
- One or more equations did not get rendered due to their size.
Instances For
ContinuousAlternatingMap.pi
as a LinearIsometryEquiv
.
Equations
- ContinuousAlternatingMap.piLIE 𝕜 E = { toLinearEquiv := ContinuousAlternatingMap.piLinearEquiv, norm_map' := ⋯ }
Instances For
ContinuousAlternatingMap.restrictScalars
as a LinearIsometry
.
Equations
- ContinuousAlternatingMap.restrictScalarsLI 𝕜' = { toFun := ContinuousAlternatingMap.restrictScalars 𝕜', map_add' := ⋯, map_smul' := ⋯, norm_map' := ⋯ }
Instances For
The difference f m₁ - f m₂
is controlled in terms of ‖f‖
and ‖m₁ - m₂‖
, precise version.
For a less precise but more usable version, see norm_image_sub_le
. The bound reads
‖f m - f m'‖ ≤ ‖f‖ * ‖m 1 - m' 1‖ * max ‖m 2‖ ‖m' 2‖ * max ‖m 3‖ ‖m' 3‖ * ... * max ‖m n‖ ‖m' n‖ + ...
,
where the other terms in the sum are the same products where 1
is replaced by any i
.
The difference f m₁ - f m₂
is controlled in terms of ‖f‖
and ‖m₁ - m₂‖
,
less precise version.
For a more precise but less usable version, see norm_image_sub_le'
.
The bound is ‖f m - f m'‖ ≤ ‖f‖ * card ι * ‖m - m'‖ * (max ‖m‖ ‖m'‖) ^ (card ι - 1)
.
If a continuous alternating map is constructed from a alternating map via the constructor
mkContinuous
, then its norm is bounded by the bound given to the constructor if it is
nonnegative.
If a continuous alternating map is constructed from a alternating map via the constructor
mk_continuous
, then its norm is bounded by the bound given to the constructor if it is
nonnegative.
ContinuousLinearMap.compContinuousAlternatingMap
as a bundled continuous bilinear map.
Equations
Instances For
ContinuousLinearMap.compContinuousAlternatingMap
as a bundled continuous linear equiv.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Flip arguments in f : F →L[𝕜] E [⋀^ι]→L[𝕜] G
to get ⋀^ι⟮𝕜; E; F →L[𝕜] G⟯
Equations
- f.flipAlternating = { toContinuousMultilinearMap := ((ContinuousAlternatingMap.toContinuousMultilinearMapCLM 𝕜).comp f).flipMultilinear, map_eq_zero_of_eq' := ⋯ }
Instances For
Composition of a continuous alternating map and a continuous linear map as a bundled continuous linear map.
Equations
Instances For
Given a continuous linear isomorphism between the domains, generate a continuous linear isomorphism between the spaces of continuous alternating maps.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Continuous linear equivalences between the domains and the codomains generate a continuous linear equivalence between the spaces of continuous alternating maps.
Equations
Instances For
Given a map f : F →ₗ[𝕜] E [⋀^ι]→ₗ[𝕜] G
and an estimate
H : ∀ x m, ‖f x m‖ ≤ C * ‖x‖ * ∏ i, ‖m i‖
, construct a continuous linear
map from F
to E [⋀^ι]→L[𝕜] G
.
In order to lift, e.g., a map f : (E [⋀^ι]→ₗ[𝕜] F) →ₗ[𝕜] E' [⋀^ι]→ₗ[𝕜] G
to a map (E [⋀^ι]→L[𝕜] F) →L[𝕜] E' [⋀^ι]→L[𝕜] G
,
one can apply this construction to f.comp ContinuousAlternatingMap.toAlternatingMapLinear
which is a linear map from E [⋀^ι]→L[𝕜] F
to E' [⋀^ι]→ₗ[𝕜] G
.
Equations
- AlternatingMap.mkContinuousLinear f C H = { toFun := fun (x : F) => (f x).mkContinuous (C * ‖x‖) ⋯, map_add' := ⋯, map_smul' := ⋯ }.mkContinuous (max C 0) ⋯
Instances For
Given a map f : E [⋀^ι]→ₗ[𝕜] (F [⋀^ι']→ₗ[𝕜] G)
and an estimate
H : ∀ m m', ‖f m m'‖ ≤ C * ∏ i, ‖m i‖ * ∏ i, ‖m' i‖
, upgrade all AlternatingMap
s in the type
to ContinuousAlternatingMap
s.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Results that are only true if the target space is a NormedAddCommGroup
(and not just a SeminormedAddCommGroup
).
Continuous alternating maps themselves form a normed group with respect to the operator norm.
If an alternating map in finitely many variables on a normed space satisfies the inequality
‖f m‖ ≤ C * ∏ i, ‖m i‖
on a shell ε i / ‖c i‖ < ‖m i‖ < ε i
for some positive numbers ε i
and elements c i : 𝕜
, 1 < ‖c i‖
, then it satisfies this inequality for all m
.