manifolds and cobordisms
cobordism theory, Introduction
Definitions
Genera and invariants
Classification
Theorems
synthetic differential geometry
Introductions
from point-set topology to differentiable manifolds
geometry of physics: coordinate systems, smooth spaces, manifolds, smooth homotopy types, supergeometry
Differentials
Tangency
The magic algebraic facts
Theorems
Axiomatics
(shape modality $\dashv$ flat modality $\dashv$ sharp modality)
$(\esh \dashv \flat \dashv \sharp )$
dR-shape modality$\dashv$ dR-flat modality
$\esh_{dR} \dashv \flat_{dR}$
(reduction modality $\dashv$ infinitesimal shape modality $\dashv$ infinitesimal flat modality)
$(\Re \dashv \Im \dashv \&)$
fermionic modality$\dashv$ bosonic modality $\dashv$ rheonomy modality
$(\rightrightarrows \dashv \rightsquigarrow \dashv Rh)$
Models
Models for Smooth Infinitesimal Analysis
smooth algebra ($C^\infty$-ring)
differential equations, variational calculus
Chern-Weil theory, ∞-Chern-Weil theory
Cartan geometry (super, higher)
For $i : X \hookrightarrow Y$ an embedding of manifolds, a tubular neighbourhood of $X$ in $Y$ is
a real vector bundle $E \to X$;
an extension of $i$ to an isomorphism
with an open neighbourhood of $X$ in $Y$.
The derivative of $\hat i$ provides an isomorphism of $E$ with the normal bundle $\nu_{X/Y}$ of $X$ in $Y$.
(tubular neighbourhood theorem)
Every embedding of smooth manifolds does admit a tubular neighbourhood, def. .
For instance (DaSilva, theorem 3.1).
Moreover, tubular neighbourhoods are unique up to homotopy in a suitable sense:
For an embedding $i : X \to Y$, write $Tub(i)$ for the topological space whose underlying set is the set of tubular neighbourhoods of $i$ and whose topology is the subspace topology of $Hom(N_i X, Y)$ equipped with the C-infinity topology.
If $X$ and $Y$ are compact manifolds, then $Tub(i)$ is contractible for all embeddings $i : X \to Y$.
This appears as (Godin, prop. 31).
These statements generalize to equivariant differential topology:
(fixed loci of smooth proper actions are submanifolds)
Let $X$ be a smooth manifold, $G$ a Lie group and $\rho \;\colon\; G \times X \to X$ a proper action by diffeomorphisms.
Then the $G$-fixed locus $X^G \hookrightarrow X$ is a smooth submanifold.
If in addition $X$ is equipped with a Riemannian metric and $G$ acts by isometries then the submanifold $X^G$ is a totally geodesic submanifold.
(e.g. Ziller 13, theorem 3.5.2, see also this MO discussion)
Let $x \in X^G \subset X$ be any fixed point. Since this is in particular a closed invariant submanifold, Prop. applies and shows that an open neighbourhood of $x$ in $X$ is $G$-equivariantly diffeomorphic to a linear representation $V \in RO(G)$. The fixed locus $V^G \subset V$ of that is hence diffeomorphic to an open neighbourhood of $x$ in $\Sigma$.
Without the assumption of proper action in Prop. the conclusion generally fails. See this MO comment for a counter-example.
($G$-action on normal bundle to fixed locus)
Let $X$ be a smooth manifold, $G$ a Lie group and $\rho \;\colon\; G \times X \to X$ a proper action by diffeomorphisms.
Then linearization of the $G$-action aroujnd the fixed locus $X^G \subset X$ equips the normal bundle $N_X\left( X^G\right)$ with smooth and fiber-wise linear $G$-action.
(e.g. Crainic-Struchiner 13, Example 1.7)
(existence of $G$-invariant Riemannian metric)
Let $X$ be a smooth manifold, $G$ a compact Lie group and $\rho \;\colon\; G \times X \to X$ a proper action by diffeomorphisms.
Then there exists a Riemannian metric on $X$ with its invariant with respect to the $G$-action, hence such that all elements of $G$ act by isometries.
(Bredon 72,VI Theorem 2.1, see also Ziller 13, Theorem 3.0.2)
($G$-equivariant tubular neighbourhood)
Let $X$ be a smooth manifold, $G$ a Lie group and $\rho \;\colon\; G \times X \to X$ a proper action by diffeomorphisms.
For $\Sigma \subset X^G \subset X$ a closed smooth submanifold inside the fixed locus, a $G$-invariant tubular neighbourhood $\mathcal{N}(\Sigma \subset X)$ of $\Sigma$ in $X$ is
a smooth vector bundle $E \to \Sigma$ equipped with a fiber-wise linear $G$-action;
an equivariant diffeomorphism $E \overset{}{\longrightarrow} X$ onto an open neighbourhood of $\Sigma$ in $X$ which takes the zero section identically to $\Sigma$.
(existence of $G$-equivariant tubular neighbourhoods)
Let $X$ be a smooth manifold, $G$ a Lie group and $\rho \;\colon\; G \times X \to X$ a proper action by diffeomorphisms.
If $\Sigma \overset{\iota}{\hookrightarrow} X$ is a closed smooth submanifold inside the $G$-fixed locus
then
$\Sigma$ admits a $G$-invariant tubular neighbourhood $\Sigma \subset U \subset X$ (Def. );
any two choices of such $G$-invariant tubular neighbourhoods are $G$-equivariantly isotopic;
there always exists an $G$-invariant tubular neighbourhood parametrized specifically by the normal bundle $N(\Sigma \subset X)$ of $Sigma$ in $X$, equipped with its induced $G$-action from Def. , and such that the $G$-equivariant diffeomorphism is given by the exponential map
with respect to a $G$-invariant Riemannian metric (which exists according to Prop. ):
The existence of the $G$-invariant tubular neighbourhoods is for instance in Bredon 72 VI Theorem 2.2, Kankaanrinta 07, theorem 4.4. The uniqueness up to equivariant isotopy is in Kankaanrinta 07, theorem 4.4, theorem 4.6. The fact that one may always use the normal bundle appears at the end of the proof of Bredon 72 VI Theorem 2.2, and as a special case of a more general statement about invariant tubular neighbourhoods in Lie groupoids it follows from Pflaum-Posthuma-Tang 11, Theorem 4.1 by applying the construction there to each point in $\Sigma$ for one and the same choice of background metric. See also for instance Pflaum-Wilkin 17, Example 2.5.
(…) propagating flow (…) (Godin).
key application: Pontrjagin-Thom collapse map
Basics on tubular neighbourhoods are reviewed for instance in
Ana Cannas da Silva, section 3 of Prerequisites from differential geometry (pdf)
Stanley Kochman, section 1.2 of Bordism, Stable Homotopy and Adams Spectral Sequences, AMS 1996
Discussion in the generality of equivariant differential topology includes
Glen Bredon, Chapter VI.2 of Introduction to compact transformation groups, Academic Press 1972 (pdf)
Glen Bredon, Introduction to compact transformation groups, Academic Press 1972 (pdf)
Marja Kankaanrinta, Equivariant collaring, tubular neighbourhood and gluing theorems for proper Lie group actions, Algebr. Geom. Topol. Volume 7, Number 1 (2007), 1-27 (euclid:agt/1513796653)
Markus Pflaum, Hessel Posthuma, X. Tang, Geometry of orbit spaces of proper Lie groupoids, Journal für die reine und angewandte Mathematik (Crelles Journal) 2014.694 (arXiv:1101.0180, doi:10.1515/crelle-2012-0092)
Wolfgang Ziller, Group actions, 2013 (pdf)
Marius Crainic, Ivan Struchiner, On the linearization theorem for proper Lie groupoids, Annales scientifiques de l’École Normale Supérieure, Série 4, Volume 46 (2013) no. 5, p. 723-746 (numdam:ASENS_2013_4_46_5_723_0 doi:10.24033/asens.2200)
Markus Pflaum, Graeme Wilkin, Equivariant control data and neighborhood deformation retractions, Methods and Applications of Analysis, 2019 (arXiv:1706.09539)
The homotopical uniqueness of tubular neighbourhoods is discussed in
For an analogue in homotopical algebraic geometry see
see also
Last revised on April 9, 2021 at 07:34:30. See the history of this page for a list of all contributions to it.