nLab stable cohomotopy





Special and general types

Special notions


Extra structure






The generalized cohomology theory which is represented by the sphere spectrum is also called stable cohomotopy, as it is the stable homotopy theory version of cohomotopy.

Equivalently, it is the cohomological dual concept to stable homotopy homology theory.

By the Pontryagin-Thom theorem this is equivalently framed cobordism cohomology theory.


As algebraic K-theory over 𝔽 1\mathbb{F}_1

The following is known as the Barratt-Priddy-Quillen theorem:


(stable cohomotopy is K-theory of FinSet)

Let 𝒞=\mathcal{C} = FinSet be a skeleton of the category of finite sets, regarded as a permutative category. Then the K-theory of this permutative category

K(FinSet)𝕊 K(FinSet) \;\simeq\; \mathbb{S}

is represented by the sphere spectrum, hence is stable cohomotopy.

This is due to Barratt-Priddy 72 reproved in Segal 74, Prop. 3.5. See also Priddy 73, Glasman 13.


(stable cohomotopy as algebraic K-theory over the field with one element)

Notice that for FF a field, the K-theory of a permutative category of its category of modules FModF Mod is its algebraic K-theory KFK F (see this example)

KFK(FMod). K F \;\simeq\; K(F Mod) \,.

Now (pointed) finite sets may be regarded as the modules over the “field with one element𝔽 1\mathbb{F}_1 (see there):

𝔽 1Mod=FinSet */ \mathbb{F}_1 Mod \;=\; FinSet^{\ast/}

If this is understood, example says that stable cohomotopy is the algebraic K-theory of the field with one element:

𝕊K𝔽 1. \mathbb{S} \;\simeq\; K \mathbb{F}_1 \,.

This perspective is highlighted in: Deitmar 06, p. 2; Guillot 06; Mahanta 17; Dundas, Goodwillie* McCarthy 13, II 1.2; Morava, Connes & Consani 16 and fully explicitly in Chu, Lorscheid & Santhanam 10, Thm. 5.9 and Beardsley & Nakamura 2024, Cor. 2.25. (Chu et al. also generalize to equivariant stable Cohomotopy and equivariant K-theory.)

(equivariant) cohomologyrepresenting
equivariant cohomology
of the point *\ast
of classifying space BGB G
ordinary cohomology
HZBorel equivariance
H G (*)H (BG,)H^\bullet_G(\ast) \simeq H^\bullet(B G, \mathbb{Z})
complex K-theory
KUrepresentation ring
KU G(*)R (G)KU_G(\ast) \simeq R_{\mathbb{C}}(G)
Atiyah-Segal completion theorem
R(G)KU G(*)compl.KU G(*)^KU(BG)R(G) \simeq KU_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {KU_G(\ast)} \simeq KU(B G)
complex cobordism cohomology
MUMU G(*)MU_G(\ast)completion theorem for complex cobordism cohomology
MU G(*)compl.MU G(*)^MU(BG)MU_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {MU_G(\ast)} \simeq MU(B G)
algebraic K-theory
K𝔽 pK \mathbb{F}_prepresentation ring
(K𝔽 p) G(*)R p(G)(K \mathbb{F}_p)_G(\ast) \simeq R_p(G)
Rector completion theorem
R 𝔽 p(G)K(𝔽 p) G(*)compl.(K𝔽 p) G(*)^Rector 73K𝔽 p(BG)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="">Rector 73</a>}}{\simeq} \!\!\!\!\!\! K \mathbb{F}_p(B G)
stable cohomotopy
K𝔽 1Segal 74K \mathbb{F}_1 \overset{\text{<a href="stable cohomotopy#StableCohomotopyIsAlgebraicKTheoryOverFieldWithOneElement">Segal 74</a>}}{\simeq} SBurnside ring
𝕊 G(*)A(G)\mathbb{S}_G(\ast) \simeq A(G)
Segal-Carlsson completion theorem
A(G)Segal 71𝕊 G(*)compl.𝕊 G(*)^Carlsson 84𝕊(BG)A(G) \overset{\text{<a href="">Segal 71</a>}}{\simeq} \mathbb{S}_G(\ast) \overset{ \text{compl.} }{\longrightarrow} \widehat {\mathbb{S}_G(\ast)} \!\! \overset{\text{<a href="">Carlsson 84</a>}}{\simeq} \!\!\!\!\!\! \mathbb{S}(B G)

The third stable framed bordism group

The third stable homotopy group of spheres is the cyclic group of order 24:

π 3 s /24 [h ] [1] \array{ \pi_3^s &\simeq& \mathbb{Z}/24 \\ [h_{\mathbb{H}}] &\leftrightarrow& [1] }

where the generator [1]/24[1] \in \mathbb{Z}/24 is represented by the quaternionic Hopf fibration S 7h S 4S^7 \overset{h_{\mathbb{H}}}{\longrightarrow} S^4.

Under the Pontrjagin-Thom isomorphism, identifying the stable homotopy groups of spheres with the bordism ring Ω fr\Omega^{fr}_\bullet of stably framed manifolds (see at MFr), this generator is represented by the 3-sphere (with its left-invariant framing induced from the identification with the Lie group SU(2) \simeq Sp(1) )

π 3 s Ω 3 fr [h ] [S 3]. \array{ \pi_3^s & \simeq & \Omega_3^{fr} \\ [h_{\mathbb{H}}] & \leftrightarrow & [S^3] \,. }

Moreover, the relation 24[S 3]02 4 [S^3] \,\simeq\, 0 is represented by the complement of 24 open balls inside the K3-manifold (MO:a/44885/381, MO:a/218053/381).

Kahn-Priddy theorem

The Kahn-Priddy theorem characterizes a comparison map between stable cohomotopy and cohomology with coefficients in the infinite real projective space P B/2\mathbb{R}P^\infty \simeq B \mathbb{Z}/2.

Boardman homomorphisms

To ordinary cohomology

Consider the unit morphism

𝕊H \mathbb{S} \longrightarrow H \mathbb{Z}

from the sphere spectrum to the Eilenberg-MacLane spectrum of the integers. For any topological space/spectrum postcomposition with this morphism induces Boardman homomorphisms of cohomology groups (in fact of commutative rings)

(1)b n:π n(X)H n(X,) b^n \;\colon\; \pi^n(X) \longrightarrow H^n(X, \mathbb{Z})

from the stable cohomotopy of XX in degree nn to its ordinary cohomology in degree nn.


(bounds on (co-)kernel of Boardman homomorphism from stable cohomotopy to integral cohomology)

If XX is a CW-spectrum which

  1. is (m1)(m-1)-(m-1)-connected

  2. of dimension dd \in \mathbb{N}


  1. the kernel of the Boardman homomorphism b nb^n (1) for

    mnd1 m \leq n\leq d -1

    is a ρ¯ dn\overline{\rho}_{d-n}-torsion group:

    ρ¯ dnker(b n)0 \overline{\rho}_{d-n} ker(b^n) \;\simeq\; 0
  2. the cokernel of the Boardman homomorphism b nb^n (1) for

    mnd2 m \leq n \leq d - 2

    is a ρ¯ dn1\overline{\rho}_{d-n-1}-torsion group:

    ρ¯ dn1coker(b n)0 \overline{\rho}_{d-n-1} coker(b^n) \;\simeq\; 0


ρ¯ i{1 | i1 j=1iexp(π j(𝕊)) | otherwise \overline{\rho}_{i} \;\coloneqq\; \left\{ \array{ 1 &\vert& i\leq 1 \\ \underoverset{j = 1}{i}{\prod} exp\left( \pi_j\left( \mathbb{S}\right) \right) &\vert& \text{otherwise} } \right.

is the product of the exponents of the stable homotopy groups of spheres in positive degree i\leq i.

(Arlettaz 04, theorem 1.2)

To topological modular forms

Write 𝕊\mathbb{S} for the sphere spectrum and tmf for the connective spectrum of topological modular forms.

Since tmf is an E-∞ring spectrum, there is an essentially unique homomorphism of E-∞ring spectra

𝕊e tmftmf. \mathbb{S} \overset{e_{tmf}}{\longrightarrow} tmf \,.

Regarded as a morphism of generalized homology-theories, this is called the Hurewicz homomorphism, or rather the Boardman homomorphism for tmftmf


(Boardman homomorphism in tmftmf is 6-connected)

The Boardman homomorphism in tmf

𝕊e tmftmf \mathbb{S} \overset{e_{tmf}}{\longrightarrow} tmf

induces an isomorphism on stable homotopy groups (hence from the stable homotopy groups of spheres to the stable homotopy groups of tmf), up to degree 6:

π 6(𝕊)π 6(e tmf)π 6(tmf). \pi_{\bullet \leq 6}(\mathbb{S}) \underoverset{\simeq}{\pi_{\bullet \leq 6}(e_{tmf})}{\longrightarrow} \pi_{\bullet\leq 6}(tmf) \,.

(Hopkins 02, Prop. 4.6, DFHH 14, Ch. 13)

flavours of
cohomology theory
(full or rational)
equivariant cohomology
(full or rational)
non-abelian cohomologyCohomotopy
(full or rational)
equivariant Cohomotopy
twisted cohomology
(full or rational)
twisted Cohomotopytwisted equivariant Cohomotopy
stable cohomology
(full or rational)
stable Cohomotopyequivariant stable Cohomotopy
differential cohomologydifferential Cohomotopyequivariant differential cohomotopy
persistent cohomologypersistent Cohomotopypersistent equivariant Cohomotopy

flavors of bordism homology theories/cobordism cohomology theories, their representing Thom spectra and cobordism rings:

bordism theory\;M(B,f) (B-bordism):

relative bordism theories:

equivariant bordism theory:

global equivariant bordism theory:



The concept of stable Cohomotopy as such:

Discussion of stable Cohomotopy as framed cobordism cohomology theory:

Discussion of stable Cohomotopy of Lie groups:

  • C. T. Stretch, Stable cohomotopy and cobordism of abelian groups, Mathematical Proceedings of the Cambridge Philosophical Society, Volume 90, Issue 2 September 1981, pp. 273-278 (doi:10.1017/S0305004100058734)

  • Ken-ichi Maruyama, ee-invariants on the stable cohomotopy groups of Lie groups, Osaka J. Math. Volume 25, Number 3 (1988), 581-589 (euclid:ojm/1200780982)

  • Sławomir Nowak, Stable cohomotopy groups of compact spaces, Fundamenta Mathematicae 180 (2003), 99-137 (doi:10.4064/fm180-2-1)

The identification of stable cohomotopy with the K-theory of the permutative category of finite sets is due to

see also:

The resulting interpretation of stable cohomotopy as algebraic K-theory over the field with one element is amplified in the following texts:

see also

The Kahn-Priddy theorem is due to

Discussion of stable Cohomotopy as framed cobordism cohomology theory:

The relation to β-rings is discussed in

  • E. Vallejo, Polynomial operations from Burnside rings to representation functors, J. Pure Appl. Algebra, 65 (1990), pp. 163–190.

  • E. Vallejo, Polynomial operations on stable cohomotopy, Manuscripta Math., 67 (1990), pp. 345–365

  • E. Vallejo, The free β-ring on one generator, J. Pure Appl. Algebra, 86 (1993), pp. 95–108.

  • Guillot 06

see also

  • Jack Morava, Rekha Santhanam, Power operations and absolute geometry (pdf)

Discussion of Boardman homomorphisms from stable cohomotopy is in

A lift of Seiberg-Witten invariants to classes in circle group-equivariant stable cohomotopy is discussed in

On (stable) motivic Cohomotopy of schemes (as motivic homotopy classes of maps into motivic Tate spheres):

Last revised on July 8, 2024 at 18:18:32. See the history of this page for a list of all contributions to it.