topology (point-set topology, point-free topology)
see also differential topology, algebraic topology, functional analysis and topological homotopy theory
Basic concepts
fiber space, space attachment
Extra stuff, structure, properties
Kolmogorov space, Hausdorff space, regular space, normal space
sequentially compact, countably compact, locally compact, sigma-compact, paracompact, countably paracompact, strongly compact
Examples
Basic statements
closed subspaces of compact Hausdorff spaces are equivalently compact subspaces
open subspaces of compact Hausdorff spaces are locally compact
compact spaces equivalently have converging subnet of every net
continuous metric space valued function on compact metric space is uniformly continuous
paracompact Hausdorff spaces equivalently admit subordinate partitions of unity
injective proper maps to locally compact spaces are equivalently the closed embeddings
locally compact and second-countable spaces are sigma-compact
Theorems
Analysis Theorems
In mathematics typically by default the term “configuration space” of a topological space $X$ refers to the topological space of pairwise distinct points in $X$, also called Fadell's configuration space, for emphasis.
In principle many other kinds of configurations and the spaces these form may be referred to by “configuration space”, notably in physics the usage is in a broader sense, see at configuration space (physics).
(configuration spaces of points)
Let $X$ be a manifold, possibly with boundary. For $n \in \mathbb{N}$, the configuration space of $n$ points in $X$ disappearing at the boundary is the topological space
where $\mathbf{\Delta}_X^n : = \{(x^i) \in X^n | \underset{i,j}{\exists} (x^i = x^j) \}$ is the fat diagonal in $X^n$ and where $\Sigma(n)$ denotes the evident action of the symmetric group by permutation of factors of $X$ inside $X^n$.
More generally, let $Y$ be another manifold, possibly with boundary. For $n \in \mathbb{N}$, the configuration space of $n$ points in $X \times Y$ vanishing at the boundary and distinct as points in $X$ is the topological space
where now $\Sigma(n)$ denotes the evident action of the symmetric group by permutation of factors of $X \times Y$ inside $X^n \times Y^n \simeq (X \times Y)^n$.
This more general definition reduces to the previous case for $Y = \ast \coloneqq \mathbb{R}^0$ being the point:
Finally the configuration space of an arbitrary number of points in $X \times Y$ vanishing at the boundary and distinct already as points of $X$ is the quotient topological space of the disjoint union space
by the equivalence relation $\sim$ given by
This is naturally a filtered topological space with filter stages
The corresponding quotient topological spaces of the filter stages reproduces the above configuration spaces of a fixed number of points:
(comparison to notation in the literature)
The above Def. is less general but possibly more suggestive than what is considered for instance in Bödigheimer 87. Concretely, we have the following translations of notation:
Notice here that when $Y$ happens to have empty boundary, $\partial Y = \emptyset$, then the pushout
is $Y$ with a disjoint basepoint attached. Notably for $Y =\ast$ the point space, we have that
is the 0-sphere.
A slight variation of the definition is sometimes useful:
(configuration space of $dim(X)$-disks)
In the situation of Def. , with $X$ a manifold of dimension $dim(X) \in \mathbb{N}$
be, on the left, the labeled configuration space of joint embeddings of tuples
of $dim(X)$-dimensional disks/closed balls into $X$, with identifications as in Def. (in particular the disks centered at the basepoint are quotiented out) and with the comparison map sending each disk to its center.
This map is evidently a deformation retraction hence in particular a homotopy equivalence.
For
$d \in \mathbb{N}$, $d \geq 1$ a natural number with $\mathbb{R}^d$ denoting the Cartesian space/Euclidean space of that dimension,
$Y$ a manifold, with non-empty boundary so that $Y / \partial Y$ is connected,
the scanning map constitutes a homotopy equivalence
between
the configuration space of arbitrary points in $\mathbb{R}^d \times Y$ vanishing at the boundary (Def. )
the $d$-fold loop space of the $d$-fold reduced suspension of the quotient space $Y / \partial Y$ (regarded as a pointed topological space with basepoint $[\partial Y]$).
In particular when $Y = \mathbb{D}^k$ is the closed ball of dimension $k \geq 1$ this gives a homotopy equivalence
with the $d$-fold loop space of the (d+k)-sphere.
(May 72, Theorem 2.7, Segal 73, Theorem 3)
(stable splitting of mapping spaces out of Euclidean space/n-spheres)
For
$d \in \mathbb{N}$, $d \geq 1$ a natural number with $\mathbb{R}^d$ denoting the Cartesian space/Euclidean space of that dimension,
$Y$ a manifold, with non-empty boundary so that $Y / \partial Y$ is connected,
there is a stable weak homotopy equivalence
between
the suspension spectrum of the configuration space of an arbitrary number of points in $\mathbb{R}^d \times Y$ vanishing at the boundary and distinct already as points of $\mathbb{R}^d$ (Def. )
the direct sum (hence: wedge sum) of suspension spectra of the configuration spaces of a fixed number of points in $\mathbb{R}^d \times Y$, vanishing at the boundary and distinct already as points in $\mathbb{R}^d$ (also Def. ).
Combined with the stabilization of the scanning map homotopy equivalence from Prop. this yields a stable weak homotopy equivalence
between the latter direct sum and the suspension spectrum of the mapping space of pointed continuous functions from the d-sphere to the $d$-fold reduced suspension of $Y / \partial Y$.
(Snaith 74, theorem 1.1, Bödigheimer 87, Example 2)
In fact by Bödigheimer 87, Example 5 this equivalence still holds with $Y$ treated on the same footing as $\mathbb{R}^d$, hence with $Conf_n(\mathbb{R}^d, Y)$ on the right replaced by $Conf_n(\mathbb{R}^d \times Y)$ in the well-adjusted notation of Def. :
Let $X= \mathbb{R}^\infty$. Then
the unordered configuration space of $n$ points in $\mathbb{R}^\infty$ is a model for the classifying space $B \Sigma(n)$ of the symmetric group $\Sigma(n)$;
(e.g. Bödigheimer 87, Example 10)
the ordered configuration space of $n$ points, equipped with the canonical $\Sigma(n)$-action, is a model for the $\Sigma(n)$-universal principal bundle.
The James construction of $X$ is homotopy equivalent to the configuration space $C(\mathbb{R}^1, X)$ of points in the real line with “charges” taking values in $X$.
(e.g. Bödigheimer 87, Example 9)
The Fulton-MacPherson compactification of configuration spaces of points in $\mathbb{R}^d$ serves to exhibit them as models for the little n-disk operad.
The stable splitting of mapping spaces says that suspension spectra of suitable mapping spaces are equivalently wedge sums of suspension spectra of configuration spaces of points.
In Euclidean field theory the correlators are often regarded as distributions of several variables with singularities on the fat diagonal. Hence they become non-singular distributions after restriction of distributions to the corresponding configuration space of points.
For more on this see at correlators as differential forms on configuration spaces of points.
General survey includes
Craig Westerland, Configuration spaces in geometry and topology, 2011 (pdf)
Ben Knudsen, Configuration spaces in algebraic topology (arXiv:1803.11165)
See also
Edward Fadell, Lee Neuwirth, Configuration spaces Math. Scand. 10 (1962) 111-118, MR141126, pdf
Edward R. Fadell, Sufian Y. Husseini, Geometry and topology of configuration spaces, Springer Monographs in Mathematics (2001), MR2002k:55038, xvi+313 pp.
F. R. Cohen, S. Gitler, On loop spaces of configuration spaces, Trans. Amer. Math. Soc. 354 (2002), no. 5, 1705–1748, MR2002m:55020
Sadok Kallel, Ines Saihi, Homotopy Groups of Diagonal Complements, Algebr. Geom. Topol. 16 (2016) 2949-2980 (arXiv:1306.6272)
The scanning map and hence the relation of configuration spaces to cohomotopy goes back to
Peter May, The geometry of iterated loop spaces, Springer 1972 (pdf)
Graeme Segal, Configuration-spaces and iterated loop-spaces, Invent. Math. 21 (1973), 213–221. MR 0331377 (pdf)
Generalization of these constructions and results is due to
Dusa McDuff, Configuration spaces of positive and negative particles, Topology Volume 14, Issue 1, March 1975, Pages 91-107 (doi:10.1016/0040-9383(75)90038-5)
Carl-Friedrich Bödigheimer, Stable splittings of mapping spaces, Algebraic topology. Springer 1987. 174-187 (pdf)
and generalization to equivariant cohomology is discussed in
See also
Sadok Kallel, Particle Spaces on Manifolds and Generalized Poincaré Dualities (arXiv:math/9810067)
Shingo Okuyama, Kazuhisa Shimakawa, Interactions of strings and equivariant homology theories, (arXiv:0903.4667)
The appearance of configuration spaces as summands in stable splittings of mapping spaces is originally due to
An alternative proof is due to
Review and generalization is in
and the relation to the Goodwillie-Taylor tower of mapping spaces is pointed out in
The configuration spaces of a space $X$ appear as the Goodwillie derivatives of its mapping space/nonabelian cohomology-functor $Maps(X,-)$:
Greg Arone, A generalization of Snaith-type filtration, Transactions of the American Mathematical Society 351.3 (1999): 1123-1150. (pdf)
Michael Ching, Calculus of Functors and Configuration Spaces, Conference on Pure and Applied Topology Isle of Skye, Scotland, 21-25 June, 2005 (pdf)
A compactification of configuration spaces of points was introduced in
and an operad-structure defined on it (Fulton-MacPherson operad) in
Review includes
This underlies the models of configuration spaces by graph complexes, see there for more.
{ReferencesCohomology}
General discussion of ordinary homology/ordinary cohomology of configuration spaces of points includes
Carl-Friedrich Bödigheimer, F. Cohen, L. Taylor, On the homology of configuration spaces, Topology Vol. 28 No. 1, p. 111-123 1989 (pdf)
Ben Knudsen, Betti numbers and stability for configuration spaces via factorization homology, Algebr. Geom. Topol. 17 (2017) 3137-3187 (arXiv:1405.6696)
Christoph Schiessl, Cohomology of Configuration Spaces (pdf)
That the de Rham cohomology of (the Fulton-MacPherson compactification of) configuration spaces of points may be modeled by graph complexes (exhibiting formality of the little n-disk operad) is due to
niecely reviewed in
Further discussion of the graph complex as a model for the de Rham cohomology of configuration spaces of points is in
Ricardo Campos, Thomas Willwacher, A model for configuration spaces of points (arXiv:1604.02043)
Ricardo Campos, Najib Idrissi, Pascal Lambrechts, Thomas Willwacher, Configuration Spaces of Manifolds with Boundary (arXiv:1802.00716)
Ricardo Campos, Julien Ducoulombier, Najib Idrissi, Thomas Willwacher, A model for framed configuration spaces of points (arXiv:1807.08319)
Last revised on November 16, 2018 at 08:59:45. See the history of this page for a list of all contributions to it.