Graded Lie algebra: Difference between revisions
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In [[mathematics]], in particular in [[functional analysis]] and [[nonlinear analysis]], it is possible to define the [[derivative (generalizations)|derivative]] of a function between two [[Fréchet space]]s. This notion of differentiation is significantly weaker than the [[Fréchet derivative|derivative in a Banach space]]. Nevertheless, it is the weakest notion of differentiation for which many of the familiar theorems from [[calculus]] hold. In particular, the [[chain rule]] is true. With some additional constraints on the Fréchet spaces and functions involved, there is an analog of the [[inverse function theorem]] called the [[Nash–Moser inverse function theorem]], having wide applications in nonlinear analysis and [[differential geometry]]. | |||
== Mathematical details == | |||
Formally, the definition of differentiation is identical to the [[Gâteaux derivative]]. Specifically, let ''X'' and ''Y'' be Fréchet spaces, ''U'' ⊂ ''X'' be an [[open set]], and ''F'' : ''U'' → ''Y'' be a function. The directional derivative of ''F'' in the direction ''v'' ∈ ''X'' is defined by | |||
:<math> | |||
DF(u)v=\lim_{\tau\rightarrow 0}\frac{F(u+v \tau)-F(u)}{\tau} | |||
</math> | |||
if the limit exists. One says that ''F'' is continuously differentiable, or ''C''<sup>1</sup> if the limit exists for all ''v'' ∈ ''X'' and the mapping | |||
:''DF'':''U'' x ''X'' → ''Y'' | |||
is a [[continuous (topology)|continuous]] map. | |||
Higher order derivatives are defined inductively via | |||
:<math>D^{k+1}F(u)\{v_1,v_2,\dots,v_{k+1}\} = \lim_{\tau\rightarrow 0}\frac{D^kF(u+\tau v_{k+1})\{v_1,\dots,v_k\}-D^kF(u)\{v_1,\dots,v_k\}}{\tau}.</math> | |||
A function is said to be ''C''<sup>k</sup> if ''D''<sup>k</sup>''F'' : ''U'' x ''X'' x ''X''x ... x ''X'' → ''Y'' is continuous. It is ''C''<sup>∞</sup>, or '''smooth''' if it is ''C''<sup>k</sup> for every ''k''. | |||
== Properties == | |||
Let ''X'', ''Y'', and ''Z'' be Fréchet spaces. Suppose that ''U'' is an open subset of ''X'', ''V'' is an open subset of ''Y'', and ''F'' : ''U'' → ''V'', ''G'' : ''V'' → ''Z'' are a pair of ''C''<sup>1</sup> functions. Then the following properties hold: | |||
* ('''Fundamental theorem of calculus'''.) | |||
::If the line segment from ''a'' to ''b'' lies entirely within ''U'', then | |||
::<math> F(b)-F(a) = \int_0^1 DF(a+(b-a)t)\cdot (b-a) dt</math>. | |||
* ('''The chain rule'''.) | |||
::''D''(''G'' o ''F'')(''u'')''x'' = ''DG''(''F''(''u''))''DF''(''u'')''x'' for all ''u'' ε ''U'' and ''x'' ε ''X''. | |||
* ('''Linearity'''.) | |||
::''DF''(''u'')''x'' is linear in ''x''.{{citation needed|date=March 2013}} More generally, if ''F'' is ''C''<sup>k</sup>, then ''DF''(''u''){''x''<sub>1</sub>,...,''x''<sub>k</sub>} is multilinear in the x's. | |||
* ('''Taylor's theorem with remainder.''') | |||
::Suppose that the line segment between ''u'' ε ''U'' and ''u+h'' lies entirely within ''u''. If ''F'' is ''C''<sup>k</sup> then | |||
::<math>F(u+h)=F(u)+DF(u)h+\frac{1}{2!}D^2F(u)\{h,h\}+\dots+\frac{1}{(k-1)!}D^{k-1}F(u)\{h,h,\dots,h\}+R_k</math> | |||
::where the remainder term is given by | |||
::<math>R_k(u,h)=\frac{1}{(k-1)!}\int_0^1(1-t)^{k-1}D^kF(u+th)\{h,h,\dots,h\}dt</math> | |||
* ('''Commutativity of directional derivatives.''') If ''F'' is ''C''<sup>k</sup>, then | |||
::<math>D^kF(u)\{h_1,...,h_k\}=D^kF(u)\{h_{\sigma(1)},\dots,h_{\sigma(k)}\}</math> for every [[permutation]] σ of {1,2,...,k}. | |||
The proofs of many of these properties rely fundamentally on the fact that it is possible to define the [[Riemann integral]] of continuous curves in a Fréchet space. | |||
==Consequences in differential geometry== | |||
The existence of a chain rule allows for the definition of a [[manifold (mathematics)|manifold]] modeled on a Frèchet space: a [[Fréchet manifold]]. Furthermore, the linearity of the derivative implies that there is an analog of the [[tangent bundle]] for Fréchet manifolds. | |||
==Tame Fréchet spaces== | |||
Frequently the Fréchet spaces that arise in practical applications of the derivative enjoy an additional property: they are '''tame'''. Roughly speaking, a tame Fréchet space is one which is almost a [[Banach space]]. On tame spaces, it is possible to define a preferred class of mappings, known as tame maps. On the category of tame spaces under tame maps, the underlying topology is strong enough to support a fully fledged theory of [[differential topology]]. Within this context, many more techniques from calculus hold. In particular, there are versions of the inverse and implicit function theorems. | |||
==References== | |||
# {{cite journal|author=Hamilton, R. S.|authorlink=Richard Hamilton (professor)|title=The inverse function theorem of Nash and Moser|url=http://projecteuclid.org/euclid.bams/1183549049| | |||
journal=Bull. AMS.|issue=7|year=1982|pages=65–222|doi=10.1090/S0273-0979-1982-15004-2|volume=7|mr=656198}} | |||
{{Functional Analysis}} | |||
{{DEFAULTSORT:Differentiation in Frechet spaces}} | |||
[[Category:Differential calculus]] | |||
[[Category:Generalizations of the derivative]] | |||
[[Category:Topological vector spaces]] |
Revision as of 02:03, 4 February 2014
In mathematics, in particular in functional analysis and nonlinear analysis, it is possible to define the derivative of a function between two Fréchet spaces. This notion of differentiation is significantly weaker than the derivative in a Banach space. Nevertheless, it is the weakest notion of differentiation for which many of the familiar theorems from calculus hold. In particular, the chain rule is true. With some additional constraints on the Fréchet spaces and functions involved, there is an analog of the inverse function theorem called the Nash–Moser inverse function theorem, having wide applications in nonlinear analysis and differential geometry.
Mathematical details
Formally, the definition of differentiation is identical to the Gâteaux derivative. Specifically, let X and Y be Fréchet spaces, U ⊂ X be an open set, and F : U → Y be a function. The directional derivative of F in the direction v ∈ X is defined by
if the limit exists. One says that F is continuously differentiable, or C1 if the limit exists for all v ∈ X and the mapping
- DF:U x X → Y
is a continuous map.
Higher order derivatives are defined inductively via
A function is said to be Ck if DkF : U x X x Xx ... x X → Y is continuous. It is C∞, or smooth if it is Ck for every k.
Properties
Let X, Y, and Z be Fréchet spaces. Suppose that U is an open subset of X, V is an open subset of Y, and F : U → V, G : V → Z are a pair of C1 functions. Then the following properties hold:
- (Fundamental theorem of calculus.)
- (The chain rule.)
- D(G o F)(u)x = DG(F(u))DF(u)x for all u ε U and x ε X.
- (Linearity.)
- DF(u)x is linear in x.Potter or Ceramic Artist Truman Bedell from Rexton, has interests which include ceramics, best property developers in singapore developers in singapore and scrabble. Was especially enthused after visiting Alejandro de Humboldt National Park. More generally, if F is Ck, then DF(u){x1,...,xk} is multilinear in the x's.
- (Taylor's theorem with remainder.)
- (Commutativity of directional derivatives.) If F is Ck, then
- for every permutation σ of {1,2,...,k}.
The proofs of many of these properties rely fundamentally on the fact that it is possible to define the Riemann integral of continuous curves in a Fréchet space.
Consequences in differential geometry
The existence of a chain rule allows for the definition of a manifold modeled on a Frèchet space: a Fréchet manifold. Furthermore, the linearity of the derivative implies that there is an analog of the tangent bundle for Fréchet manifolds.
Tame Fréchet spaces
Frequently the Fréchet spaces that arise in practical applications of the derivative enjoy an additional property: they are tame. Roughly speaking, a tame Fréchet space is one which is almost a Banach space. On tame spaces, it is possible to define a preferred class of mappings, known as tame maps. On the category of tame spaces under tame maps, the underlying topology is strong enough to support a fully fledged theory of differential topology. Within this context, many more techniques from calculus hold. In particular, there are versions of the inverse and implicit function theorems.
References
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