Asymptotics in a Topological Vector Space #
This file defines Asymptotics.IsLittleOTVS, Asymptotics.IsBigOTVS, and Asymptotics.IsThetaTVS
as generalizations of Asymptotics.IsLittleO, Asymptotics.IsBigO, and Asymptotics.IsTheta
from normed spaces to topological vector spaces.
Given two functions f and g taking values in topological vector spaces
over a normed field K,
we say that $f = o(g)$ (resp., $f = O(g)$)
if for any neighborhood of zero U in the codomain of f
there exists a neighborhood of zero V in the codomain of g
such that $\operatorname{gauge}_{K, U} (f(x)) = o(\operatorname{gauge}_{K, V} (g(x)))$
(resp., $\operatorname{gauge}_{K, U} (f(x)) = O(\operatorname{gauge}_{K, V} (g(x)))$),
where $\operatorname{gauge}_{K, U}(y) = \inf \{‖c‖ \mid y ∈ c • U\}$.
We say that $f=Θ(g)$, if both $f=O(g)$ and $g=O(f)$.
In a normed space, we can use balls of positive radius as both U and V,
thus reducing the definition to the classical one.
These modifications of the definitions free the user from having to chose a canonical norm,
at the expense of having to pick a specific base field.
This is exactly the tradeoff we want in HasFDerivAtFilter,
as there the base field is already chosen,
and this removes the choice of norm being part of the statement.
These definitions were added to the library in order to migrate Fréchet derivatives
from normed vector spaces to topological vector spaces.
The definitions are motivated by
https://en.wikipedia.org/wiki/Fr%C3%A9chet_derivative#Generalization_to_topological_vector_spaces
but the definition there doesn't work for topological vector spaces over general normed fields.
This Zulip discussion
led to the current choice of the definition of Asymptotics.IsLittleOTVS,
and Asymptotics.IsBigOTVS was defined in a similar manner.
Main results #
isLittleOTVS_iff_isLittleO: the equivalence between these two definitions in the case of a normed space.isLittleOTVS_iff_tendsto_inv_smul: the equivalence to convergence of the ratio to zero in case of a topological vector space.
TODO #
- Add
Asymptotics.IsEquivalentTVS. - Prove a version of
Asymptotics.isBigO_oneforIsBigOTVS.
f =o[𝕜; l] g (IsLittleOTVS 𝕜 l f g) is a generalization of f =o[l] g (IsLittleO l f g)
that works in topological 𝕜-vector spaces.
Given two functions f and g taking values in topological vector spaces
over a normed field K,
we say that $f = o(g)$ if for any neighborhood of zero U in the codomain of f
there exists a neighborhood of zero V in the codomain of g
such that $\operatorname{gauge}_{K, U} (f(x)) = o(\operatorname{gauge}_{K, V} (g(x)))$,
where $\operatorname{gauge}_{K, U}(y) = \inf \{‖c‖ \mid y ∈ c • U\}$.
We use an ENNReal-valued function egauge for the gauge,
so we unfold the definition of little o instead of reusing it.
Instances For
f =o[𝕜; l] g (IsLittleOTVS 𝕜 l f g) is a generalization of f =o[l] g (IsLittleO l f g)
that works in topological 𝕜-vector spaces.
Given two functions f and g taking values in topological vector spaces
over a normed field K,
we say that $f = o(g)$ if for any neighborhood of zero U in the codomain of f
there exists a neighborhood of zero V in the codomain of g
such that $\operatorname{gauge}_{K, U} (f(x)) = o(\operatorname{gauge}_{K, V} (g(x)))$,
where $\operatorname{gauge}_{K, U}(y) = \inf \{‖c‖ \mid y ∈ c • U\}$.
We use an ENNReal-valued function egauge for the gauge,
so we unfold the definition of little o instead of reusing it.
Equations
- One or more equations did not get rendered due to their size.
Instances For
f =O[𝕜; l] g (IsBigOTVS 𝕜 l f g) is a generalization of f =O[l] g (IsBigO l f g)
that works in topological 𝕜-vector spaces.
Given two functions f and g taking values in topological vector spaces
over a normed field 𝕜,
we say that $f = O(g)$ if for any neighborhood of zero U in the codomain of f
there exists a neighborhood of zero V in the codomain of g
such that $\operatorname{gauge}_{K, U} (f(x)) \le \operatorname{gauge}_{K, V} (g(x))$,
where $\operatorname{gauge}_{K, U}(y) = \inf \{‖c‖ \mid y ∈ c • U\}$.
Instances For
f =O[𝕜; l] g (IsBigOTVS 𝕜 l f g) is a generalization of f =O[l] g (IsBigO l f g)
that works in topological 𝕜-vector spaces.
Given two functions f and g taking values in topological vector spaces
over a normed field 𝕜,
we say that $f = O(g)$ if for any neighborhood of zero U in the codomain of f
there exists a neighborhood of zero V in the codomain of g
such that $\operatorname{gauge}_{K, U} (f(x)) \le \operatorname{gauge}_{K, V} (g(x))$,
where $\operatorname{gauge}_{K, U}(y) = \inf \{‖c‖ \mid y ∈ c • U\}$.
Equations
- One or more equations did not get rendered due to their size.
Instances For
We say that f =Θ[𝕜; l] g (IsThetaTVS 𝕜 l f g), if f =O[𝕜; l] g and g =O[𝕜; l] f.
It is a generalization of f =Θ[l] g that works in topological 𝕜-vector spaces.
Instances For
We say that f =Θ[𝕜; l] g (IsThetaTVS 𝕜 l f g), if f =O[𝕜; l] g and g =O[𝕜; l] f.
It is a generalization of f =Θ[l] g that works in topological 𝕜-vector spaces.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Alias of the forward direction of Asymptotics.isLittleOTVS_iff_tendsto_div.
Alias of the reverse direction of Asymptotics.isLittleOTVS_iff_tendsto_div.
A version of IsLittleOTVS.exists_eventuallyLE_mul
where ε is quantified over ℝ≥0∞ instead of ℝ≥0.
A stronger version of IsLittleOTVS.congr that requires the functions only agree along the
filter.
A stronger version of IsBigOTVS.congr that requires the functions only agree along the
filter.
Transitivity lemmas #
Equations
- Asymptotics.instTransIsBigOTVSIsBigOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsBigOTVSIsThetaTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsThetaOTVSIsBigOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsThetaOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsLittleOTVSIsBigOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsLittleOTVSIsThetaTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsBigOTVSIsLittleOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsThetaTVSIsLittleOTVS = { trans := ⋯ }
Equations
- Asymptotics.instTransIsLittleOTVSIsLittleOTVS = { trans := ⋯ }
The definition of IsBigOTVS says that
for each neighborhood U of the origin in the codomain of f,
there exists a neighborhood V of the origin in the codomain of g such that
egauge 𝕜 U (f x) ≤ egauge 𝕜 V (g x) eventually along l.
This lemma shows that it suffices to make this inequality work up to a constant multiplier.
Alias of the forward direction of Asymptotics.isLittleOTVS_iff_smallSets.
Alias of the forward direction of Asymptotics.isBigOTVS_iff_smallSets.
Eta-expanded form of Asymptotics.IsLittleOTVS.symm
Eta-expanded form of Asymptotics.IsBigOTVS.symm
If f converges along l to a finite limit x, then f =O[𝕜, l] 1.
Alias of the reverse direction of Asymptotics.isLittleOTVS_iff_isLittleO.
Alias of the forward direction of Asymptotics.isLittleOTVS_iff_isLittleO.
Alias of the forward direction of Asymptotics.isBigOTVS_iff_isBigO.
Alias of the reverse direction of Asymptotics.isBigOTVS_iff_isBigO.