nLab Kahn-Priddy theorem





Special and general types

Special notions


Extra structure





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


Write P Ho(Top)\mathbb{R}P^\infty \in Ho(Top) for the homotopy type of real projective space (an object in the classical homotopy category), and write Σ P + Ho(Spectra)\Sigma^\infty \mathbb{R}P^\infty_+ \in Ho(Spectra) for its suspension spectrum regarded as an H-group ring spectrum in the stable homotopy category.

For each nn there is a canonical inclusion (see Whitehead 83, p. 20).

P n1O(n) + \mathbb{R}P^{n-1} \hookrightarrow O(n)^+

due to Hopf 35, which is compatible with the inclusions as nn varies

P n1 O(n) P n O(n+1) \array{ \mathbb{R}P^{n-1} &\hookrightarrow& O(n) \\ \downarrow && \downarrow \\ \mathbb{R}P^n &\hookrightarrow& O(n+1) }

and hence induces an inclusion

P O \mathbb{R}P^\infty \hookrightarrow O


Composing this with the J-homomorphism gives a map

ϕ:Σ P 𝕊 \phi \;\colon\; \Sigma^\infty \mathbb{R}P^\infty \longrightarrow \mathbb{S}

from the H-group ring spectrum of infnite real projective space to the sphere spectrum.

Then for XX a connected 2-primary finite CW-complex, the function that takes stable maps into this H-group ring spectrum to maps to the sphere spectrum, hence to the stable cohomotopy of XX

ϕ X:[Σ X +,Σ P + ]surj.[Σ X +,𝕊] \phi_X \;\colon\; [\Sigma^\infty X_+ , \Sigma^\infty \mathbb{R}P^\infty_+ ] \overset{surj.}{\longrightarrow} [\Sigma^\infty X_+, \mathbb{S}]

is surjective.

In this form this is stated in Adams 73, lemma 3.1 (see the notation introduced below lemma 2.2: for p=2p = 2 then Adams’s LL is P \mathbb{R}P^\infty).

Analog for complex projective space

The statement was announced in the above form in Segal 73, prop. 2, where the analogous statement for complex projective space and topological K-theory is proven (see this prop.):

[Σ X +,Σ P + ]surj.K (X). [\Sigma^\infty X_+ , \Sigma^\infty \mathbb{C}P^\infty_+ ] \overset{surj.}{\longrightarrow} K_{\mathbb{C}}(X) \,.

Notice that Snaith's theorem asserts that this map becomes in fact an isomorphism to reduced K-theory after quotienting out the Bott generator βΣ P + \beta \in \Sigma^\infty \mathbb{C}P^\infty_+.


The original formulation is due to

A strengthening was obtained in

  • John Frank Adams, The Kahn-Priddy theorem, Mathematical Proceedings of the Cambridge Philosophical Society, Mathematical Proceedings of the Cambridge Philosophical Society 55, 1973

Review is in

  • George Whitehead, pages 20, 21 of Fifty years of homotopy, Bulletin of the AMS, Volume 8, Number 1, 1983 (pdf)

The analogous statement for complex projective space and complex topological K-theory is due to

Last revised on September 10, 2018 at 08:16:02. See the history of this page for a list of all contributions to it.