homotopy theory, (∞,1)-category theory, homotopy type theory
flavors: stable, equivariant, rational, p-adic, proper, geometric, cohesive, directed…
models: topological, simplicial, localic, …
see also algebraic topology
Introductions
Definitions
Paths and cylinders
Homotopy groups
Basic facts
Theorems
group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
manifolds and cobordisms
cobordism theory, Introduction
Where homotopy groups are groups of homotopy classes of maps out spheres, $\pi_n(X)\coloneqq [S^n \to X]$, cohomotopy groups are groups of homotopy classes into spheres, $\pi^n(X) \coloneqq [X \to S^n]$.
If instead one considers mapping into the stabilization of the spheres, hence into (some suspension of) the sphere spectrum, then one speaks of stable cohomotopy. In other words, the generalized (Eilenberg-Steenrod) cohomology theory which is represented by the sphere spectrum is stable cohomotopy.
In this vein, regarding terminology: the concept of cohomology (as discussed there) in the very general sense of non-abelian cohomology, is about homotopy classes of maps into any object $A$ (in some (∞,1)-topos). In this way, general non-abelian cohomology is sort of dual to homotopy, and hence might generally be called co-homotopy. This is the statement of Eckmann-Hilton duality. The duality between homotopy (groups) and co-homotopy proper may then be thought of as being the special case of this where $A$ is taken to be a sphere.
Let $n \in \mathbb{N}$ be a natural number and $X \in Mfd$ be a connected orientable closed manifold of dimension $n$. Then the $n$th cohomotopy classes $\left[X \overset{c}{\to} S^n\right] \in \pi^n(X)$ of $X$ are in bijection to the degree $deg(c) \in \mathbb{Z}$ of the representing functions, hence the canonical function
from $n$th cohomotopy to $n$th integral cohomology is a bijection.
(e.g. Kosinski 93, IX (5.8))
relation to the Freudenthal suspension theorem (Spanier 49, section 9)
For $X$ a compact smooth manifold, there is a smooth function $X \to S^n$ representing every cohomotopy class (with respect to the standard smooth structure on the sphere manifold).
For $X$ a closed smooth manifold of dimension $D$, the Pontryagin-Thom construction (e.g. Kosinski 93, IX.5) identifies the set
of cobordism classes of closed and normally framed submanifolds $\Sigma \overset{\iota}{\hookrightarrow} X$ of dimension $d$ inside $X$ with the cohomotopy $\pi^{D-d}(X)$ of $X$ in degree $D- d$
(e.g. Kosinski 93, IX Theorem (5.5))
In particular, by this bijection the canonical group structure on cobordism groups in sufficiently high codimension (essentially given by disjoint union of submanifolds) this way induces a group structure on the cohomotopy sets in sufficiently high degree.
This construction generalizes to equivariant cohomotopy, see there.
(configuration spaces and twisted cohomotopy)
The scanning map equivalence (this Prop.) identifies the configuration space of points in $X$ with labels in an n-sphere with the cocycle-space/-infinity-groupoid of $\tau_X$-twisted cohomotopy in degree $\tau + n$, where $\tau_X \coloneqq [S_X(T X)]$ is the class of the spherical fibration of the tangent bundle.
In particular if $X$ is a parallelizable manifold/framed manifold, then $\tau_X = dim(X)$ and the equivalence identifies the configuration space with the plain cohomotopy of $X$ in degree $dim(X) + n$:
Let $X$ be a 4-manifold which is connected and oriented.
The Pontryagin-Thom construction as above gives for $n \in \mathbb{Z}$ the commuting diagram of sets
where $\pi^\bullet$ denotes cohomotopy sets, $H^\bullet$ denotes ordinary cohomology, $H_\bullet$ denotes ordinary homology and $\mathbb{F}_\bullet$ is normally framed cobordism classes of normally framed submanifolds. Finally $h^n$ is the operation of pullback of the generating integral cohomology class on $S^n$ (by the nature of Eilenberg-MacLane spaces):
Now
$h^0$, $h^1$, $h^4$ are isomorphisms
$h^3$ is an isomorphism if $X$ is “odd” in that it contains at least one closed oriented surface of odd self-intersection, otherwise $h^3$ becomes an isomorphism on a $\mathbb{Z}/2$-quotient group of $\pi^3(X)$ (which is a group via the group-structure of the 3-sphere (SU(2)))
cohomology | equivariant cohomology | |
---|---|---|
non-abelian cohomology | cohomotopy | equivariant cohomotopy |
twisted cohomology | twisted cohomotopy | |
stable cohomology | stable cohomotopy | equivariant stable cohomotopy |
$\,$
(equivariant) cohomology | representing spectrum | equivariant cohomology of the point $\ast$ | cohomology of classifying space $B G$ |
---|---|---|---|
(equivariant) ordinary cohomology | HZ | Borel equivariance $H^\bullet_G(\ast) \simeq H^\bullet(B G, \mathbb{Z})$ | |
(equivariant) complex K-theory | KU | representation ring $KU_G(\ast) \simeq R_{\mathbb{C}}(G)$ | Atiyah-Segal completion theorem $R(G) \simeq KU_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {KU_G(\ast)} \simeq KU(B G)$ |
(equivariant) complex cobordism cohomology | MU | $MU_G(\ast)$ | completion theorem for complex cobordism cohomology $MU_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {MU_G(\ast)} \simeq MU(B G)$ |
(equivariant) algebraic K-theory | $K \mathbb{F}_p$ | representation ring $(K \mathbb{F}_p)_G(\ast) \simeq R_p(G)$ | Rector completion theorem $R_{\mathbb{F}_p}(G) \simeq K (\mathbb{F}_p)_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {(K \mathbb{F}_p)_G(\ast)} \!\! \overset{\text{<a href="https://ncatlab.org/nlab/show/Rector+completion+theorem">Rector 73</a>}}{\simeq} \!\!\!\!\!\! K \mathbb{F}_p(B G)$ |
(equivariant) stable cohomotopy | K $\mathbb{F}_1 \overset{\text{<a href="stable cohomotopy#StableCohomotopyIsAlgebraicKTheoryOverFieldWithOneElement">Segal 74</a>}}{\simeq}$ S | Burnside ring $\mathbb{S}_G(\ast) \simeq A(G)$ | Segal-Carlsson completion theorem $A(G) \overset{\text{<a href="https://ncatlab.org/nlab/show/Burnside+ring+is+equivariant+stable+cohomotopy+of+the+point">Segal 71</a>}}{\simeq} \mathbb{S}_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {\mathbb{S}_G(\ast)} \!\! \overset{\text{<a href="https://ncatlab.org/nlab/show/Segal-Carlsson+completion+theorem">Carlsson 84</a>}}{\simeq} \!\!\!\!\!\! \mathbb{S}(B G)$ |
Original articles include
K. Borsuk, Sur les groupes des classes de transformations continues, CR Acad. Sci. Paris 202.1400-1403 (1936): 2 (doi:10.24033/asens.603)
Edwin Spanier, Borsuk’s Cohomotopy Groups, Annals of Mathematics Second Series, Vol. 50, No. 1 (Jan., 1949), pp. 203-245 (jstor:1969362)
Franklin P. Peterson, Some Results on Cohomotopy Groups, American Journal of Mathematics Vol. 78, No. 2 (Apr., 1956), pp. 243-258 (jstor:2372514)
The relation between cohomotopy classes of manifolds to the cobordism group is discussed for instance in
Further discussion includes
Laurence Taylor, The principal fibration sequence and the second cohomotopy set, Proceedings of the Freedman Fest, 235251, Geom. Topol. Monogr., 18, Coventry, 2012 (arXiv:0910.1781)
Robion Kirby, Paul Melvin, Peter Teichner, Cohomotopy sets of 4-manifolds, GTM 18 (2012) 161-190 (arXiv:1203.1608)
See also
Wikipedia, Cohomotopy group
Discussion of the equivariant cohomology-version of cohomotopy (equivariant cohomotopy):
Arthur Wasserman, section 3 of Equivariant differential topology, Topology Vol. 8, pp. 127-150, 1969 (pdf)
James Cruickshank, Twisted homotopy theory and the geometric equivariant 1-stem, Topology and its Applications Volume 129, Issue 3, 1 April 2003, Pages 251-271 (arXiv:10.1016/S0166-8641(02)00183-9)
Daniel Grady, Cobordisms of global quotient orbifolds and an equivariant Pontrjagin-Thom construction (arXiv:1811.08794)
Discussion of M-brane physics in terms of rational equivariant cohomotopy:
and in terms of twisted cohomotopy:
Last revised on April 19, 2019 at 17:13:14. See the history of this page for a list of all contributions to it.