nLab Steenrod approximation theorem



Differential geometry

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from point-set topology to differentiable manifolds

geometry of physics: coordinate systems, smooth spaces, manifolds, smooth homotopy types, supergeometry



smooth space


The magic algebraic facts




infinitesimal cohesion

tangent cohesion

differential cohesion

graded differential cohesion

singular cohesion

id id fermionic bosonic bosonic Rh rheonomic reduced infinitesimal infinitesimal & étale cohesive ʃ discrete discrete continuous * \array{ && id &\dashv& id \\ && \vee && \vee \\ &\stackrel{fermionic}{}& \rightrightarrows &\dashv& \rightsquigarrow & \stackrel{bosonic}{} \\ && \bot && \bot \\ &\stackrel{bosonic}{} & \rightsquigarrow &\dashv& \mathrm{R}\!\!\mathrm{h} & \stackrel{rheonomic}{} \\ && \vee && \vee \\ &\stackrel{reduced}{} & \Re &\dashv& \Im & \stackrel{infinitesimal}{} \\ && \bot && \bot \\ &\stackrel{infinitesimal}{}& \Im &\dashv& \& & \stackrel{\text{étale}}{} \\ && \vee && \vee \\ &\stackrel{cohesive}{}& \esh &\dashv& \flat & \stackrel{discrete}{} \\ && \bot && \bot \\ &\stackrel{discrete}{}& \flat &\dashv& \sharp & \stackrel{continuous}{} \\ && \vee && \vee \\ && \emptyset &\dashv& \ast }


Lie theory, ∞-Lie theory

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Cartan geometry (super, higher)



The Steenrod-approximation theorem states mild conditions under which an extension of a smooth function on a closed subset by a continuous function may itself be improved to an extension by a smooth function.

This is a smooth enhancement of the Tietze extension theorem.



Let XX be a finite dimensional connected smooth manifold with corners. Let π:EX\pi : E \to X be a locally trivial smooth bundle with a locally convex manifold NN as typical fiber and σ:XE\sigma : X \to E a continuous section.

If LXL \subset X is a closed subset and UXU \subset X is an open subset such that σ\sigma is smooth in a neighbourhood of LUL \setminus U, then:

  1. for each open neighbourhood OO of σ(X)\sigma(X) in EE there exists a section τ:XO\tau : X \to O

    • which is smooth in a neighbourhood of LL;

    • and which equals σ\sigma on XUX \setminus U;

  2. there exists a homotopy F:[0,1]×XOF : [0,1] \times X \to O between σ\sigma and τ\tau such that

    • each F(t,)F(t,-) is a section of π\pi;

    • for (t,x)[0,1]×(XU)(t,x) \in [0,1] \times (X \setminus U) we have F(t,x)=σ(x)=τ(x)F(t,x) = \sigma(x) = \tau(x).

See (Wockel)

O id O id O smooth σ| XU= τ| XU σ F τ smooth LU XU X id X L \array{ && O &\stackrel{id}{\to} & O & \stackrel{id}{\to}& O \\ & {}^{\mathllap{smooth}}\nearrow & {}_{\mathllap{\sigma|_{X \setminus U}}}\uparrow = \uparrow_{\mathrlap{\tau|_{X \setminus U}}} && \uparrow^{\mathrlap{\sigma}} & \swArrow_F& \uparrow^{\exists \tau} & \nwarrow^{\mathrlap{smooth}} \\ L \setminus U &\hookrightarrow & X \setminus U &\hookrightarrow& X &\stackrel{id}{\to}& X &\stackrel{}{\hookleftarrow}& L }


Smoothing of delayed homotopies


Let f,g:ZYf,g : Z \to Y be two smooth functions between smooth manifolds. Let η:Z×[0,1]Y\eta : Z \times [0,1] \to Y be a continuous delayed homotopy between them, constant in a neighbourhood Z×([0,ϵ)(1ϵ,1])Z \times ([0,\epsilon) \coprod (1-\epsilon,1]).

Then there exists also smooth homotopy between ff and gg which is itself continuously homotopic to η\eta.


To apply the generalized Steenrod theorem with the notation as stated there, make the following identifications

  • set XZ×[0,1]X \coloneqq Z \times [0,1];

  • set N=YN = Y;

  • let E=Z×[0,1]×YE = Z \times [0,1] \times Y be the trivial ZZ-bundle over XX

    (so that sections of EE are equivalently functions Z×[0,1]YZ \times[0,1] \to Y)

  • let (σ:XE):=(η:Z×[0,1]Y)(\sigma : X \to E) := (\eta : Z \times [0,1] \to Y) be the given continuous homotopy;

  • set LZ×[0,1]L \coloneqq Z \times [0,1];

  • let UZ×(0,1)U \coloneqq Z \times (0,1).

Then because by assumption η\eta is a continuous delayed homotopy between smooth functions, it follows that σ\sigma is smooth in a neighbourhood Z×([0,ϵ)(1ϵ,1])Z \times ([0,\epsilon) \coprod (1-\epsilon,1]) of LL.

So the theorem applies and provides a smooth homotopy

τ:[0,1]×ZY \tau \colon [0,1] \times Z \to Y

which moroever is itself (continuously) homotopic to η\eta via some continuous F:[0,1]×[0,1]×ZYF : [0,1] \times [0,1] \times Z \to Y.


Last revised on June 30, 2022 at 19:59:06. See the history of this page for a list of all contributions to it.