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group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
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For $p$ a prime number, the Steenrod algebra $\mathcal{A}$ is the associative algebra over the prime field $\mathbb{F}_p$ of cohomology operations on ordinary cohomology with coefficients in $\mathbb{F}_p$. For $p = 2$ this is the $\mathbb{F}_2$-algebra generated by the Steenrod square operations. (Often the case $p = 2$ is understood by default.)
For any prime $p$, the mod-$p$ Steenrod algebra furthermore has the structure of a Hopf algebra over $\mathbb{F}_p$, non-commutative but co-commutative. Under forming linear duals this gives the dual Steenrod algebra, traditionally denoted $\mathcal{A}_\ast$ instead of $\mathcal{A}^\ast$. This is a Hopf algebra over $\mathbb{F}_p$ that is commutative, but non-co-commutative.
The dual $\mathbb{F}_p$ Steenrod algebra is a special case of a commutative Hopf algebroid structure canonically induced on the self generalized homology $E_\bullet(E)$ of any ring spectrum $E$ for which $E_\bullet \to E_\bullet(E)$ is a flat morphisms. Therefore in this general case one sometimes speaks of “dual $E$-Steenrod algebras”.
The Ext-groups between comodules for these commutative Hopf algebroids $E_\bullet(E)$ prominently appear on the second page of the $E$-Adams spectral sequence, see there for more.
(…) (e.g. Mosher-Tangora 68, section 6, Lurie 07) (…)
The Steenrod algebra and its standard properties, such as the Adem relations, follow abstractly from the Cotor groups of comodules over any commutative Hopf algebroid.
This is due to (May 70, 11.8). A review is in (Ravenel, appendix 1, theorem A1.5.2).
In particular for $E$ a suitable E-infinity ring, its self-generalized homology $E_\bullet(E)$ form a (graded-)commutative Hopf algebroid over $E_\bullet$.
See at Hopf algebroid structure – For generalized cohomology below.
The Serre-Cartan basis is the subset of elements of the Steenrod algebra on those of the form
where $i_k \in \mathbb{N}$ subject to the relation
This is indeed a linear basis for the $\mathbb{F}_2$-vector space underlying the Steenrod algebra.
The Steenrod square operations satisfy the following relation, for all for all $0 \lt i \lt 2 j$:
These are called the Ádem relations (Ádem 52).
The Ádam relations precisely generate the ideal of relations among the Serre-Cartan basis elements, def. , in the Steenrod algebra.
More generally, for other prime numbers:
Let $p$ be a prime number. Write $\mathbb{F}_p$ for the corresponding prime field.
The mod $p$-Steenrod algebra $\mathcal{A}_{\mathbb{F}_p}$ is the graded co-commutative Hopf algebra over $\mathbb{F}_p$ which is
for $p = 2$ generated by elements denoted $Sq^n$ for $n \in \mathbb{N}$, $n \geq 1$;
for $p \gt 2$ generated by elements denoted $\beta$ and $P^n$ for $n \in \mathbb{N}$, $n \geq 1$
(called the Serre-Cartan basis elements) whose product is subject to the following relations (called the Ádem relations):
for $p = 2$:
for $0 \lt h \lt 2k$ the
for $p \gt 2$:
for $0 \lt h \lt p k$ then
and if $0 \lt h \lt p k$ then
and whose coproduct $\Psi$ is subject to the following relations:
for $p = 2$:
for $p \gt 2$:
and
e.g. (Kochmann 96, p. 52)
The $\mathbb{F}_p$-linear dual of the mod $p$-Steenrod algebra (def. ) is itself naturally a graded commutative Hopf algebra (with coproduct the linear dual of the original product, and vice versa), called the dual Steenrod algebra $\mathbb{A}_{\mathbb{F}_p}^\ast$.
There is an isomorphism
(e.g. Rognes 12, remark 7.24)
We now give the generators-and-relations description of the dual Steenrod algebra $\mathcal{A}^\ast_{\mathbb{F}_p}$ from def. , in terms of linear duals of the generators for $\mathcal{A}_{\mathbb{F}_p}$ itself, according to def. .
In the following, we use for $p = 2$ the notation
This serves to unify the expressions for $p = 2$ and for $p \gt 2$ in the following. Notice that for all $p$
$P^n$ has even degree $deg(P^n) = 2n(p-1)$;
$\beta$ has odd degree $deg(\beta) = 1$.
The dual mod $p$-Steenrod algebra $\mathcal{A}^\ast_{\mathbb{F}_p}$ (def. ) is, as an associative algebra, the free graded commutative algebra
on generators:
$\xi_n$ being the linear dual to $P^{p^{n-1}} P^{p^{n-2}} \cdots P^p P^1$;
$\tau_n$ being linear dual to $P^{p^{n-1}} P^{p^{n-2}} \cdots P^p P^1\beta$.
Moreover, the coproduct on $\mathcal{A}^\ast_{\mathbb{F}_p}$ is given by
and
where we set $\xi_0 \coloneqq 1$.
This is due to (Milnor 58). See for instance (Kochmann 96, theorem 2.5.1)
The Steenrod algebra for mod $p$ coefficients is a Hopf algebra over $\mathbb{F}_p$ which is graded commutative and non-co-commutative.
This is due to (Milnor 58). A review is in Ravenel, ch. 3, section 1.
More generally:
Let $R$ be an E-∞ ring and let $A$ an E-∞ algebra over $R$. The self-generalized homology $A^R_\bullet(A)$ is naturally a module over the cohomology ring $A_\bullet$ via applying the homotopy groups $\infty$-functor $\pi_\bullet$ to the canonical inclusion
Let $R$ be an E-∞ ring and let $A$ an E-∞ algebra over $R$. If the the $A_\bullet$-module $A^R_\bullet(A)$ of lemma is a flat module, then
$(A_\bullet, A_\bullet(A))$ is a commutative Hopf algebroid over $R_\bullet$;
$A^R_\bullet(X)$ is a left $A^R_\bullet(A)$-module for every $R$-∞-module $X$.
This is due to (Baker-Lazarev 01), further discussed in (Baker-Jeanneret 02) (there expressed in terms of the presentation by highly structured ring spectra). A review is also in (Ravenel, chapter 2, prop. 2.2.8).
Using the arguments of (Adams 74, Ravenel 86).
The flatness condition implies that there is an equivalence
Combining this with the map in lemma yields the coaction
These (dual) $E$-Steenrod algebra Hopf algebroids have also been called “brave new Hopf algebroids” (Baker, Baker-Jeanneret 02)
For $E = H \mathbb{F}_2$ the Eilenberg-MacLane spectrum, this reproduces the Hopf algebra structure on the dual ordinary Steenrod algebra as above.
For $E =$ MU then by Quillen's theorem on MU $\pi_\bullet(MU) \simeq L$ is the Lazard ring and the Hopf algebroid $(\pi_\bullet(MU), MU_\bullet(MU))$ is described by the Landweber-Novikov theorem.
For $E =$ BP the analog is the content of the Adams-Quillen theorem. The Landweber exact functor theorem was proven using the $B P_\bullet(B P)$-Hopf algebroid.
By construction, the total cohomology $H^\bullet(X,\mathbb{Z}_2)$ of a topological space $X$, naturally is a module over the Steenrod algebra:
a cohomology element is represented by a cocycle which is a map $X \longrightarrow B^n \mathbb{Z}_2$ and the action of a Steenrod square on this is just by composition.
In this form the Steenrod algebra appears in the second page of the Adams spectral sequence which computes $[\Sigma^\infty X, \Sigma^\infty Y]$ for topological spaces $X$ and $Y$: that second page is given by the Ext-groups
computed in the category of A-modules for $A$ the Steenrod algebra.
More generally, For $R$ an E-infinity ring such that its dual $R$-Steenrod algebra in the form of the self-homology $R_\bullet(R)$ is a (graded-)commutative Hopf algebroid over $R_\bullet = \pi_\bullet(R)$ (see at Steenrod algebra – Hopf algebroid structure), then the $E^2$-term of the $E$-Adams spectral sequence is an Ext of $E_\bullet(E)$-comodules
See the references below.
Original articles include
José Ádem, The iteration of the Steenrod squares in algebraic topology , Proceedings of the National Academy of Sciences of the United States of America 38: 720–726 (1952)
John Milnor, The Steenrod algebra and its dual, Ann. of Math. 67 (1958), 150–171.
Norman Steenrod, David Epstein, Cohomology operations, Ann. of Math. Studies, no. 50, Princeton University Press, Princeton, N. J., 1962.
John Frank Adams, part III, section 12 of Stable homotopy and generalised homology, University of Chicago Press (1974).
Comprehensive discussion of the ordinary Steenrod algebra, with proof is the Adem relations includes
Robert Mosher, Martin Tangora, Cohomology Operations and Application in Homotopy Theory, Harper and Row (1968) (pdf)
Jacob Lurie, 18.917 Topics in Algebraic Topology: The Sullivan Conjecture, Fall 2007. (MIT OpenCourseWare: Massachusetts Institute of Technology), Lecture notes
Lecture 2 Steenrod operations (pdf)
Lecture 3 Basic properties of Steenrod operations (pdf)
Lecture 4 The Adem relations (pdf)
Lecture 5 The Adem relations (cont.) (pdf)
The general algebraic approach was laid out in
Peter May, A general algebraic approach to Steenrod operations, in The Steenrod Algebra and its Applications (Proc. Conf. to Celebrate N. E. Steenrod’s Sixtieth Birthday, Battelle Memorial Inst., Columbus, Ohio, 1970), Lecture Notes in Mathematics, Vol. 168, Springer, Berlin, 1970, pp. 153–231. (pdf)
Robert Bruner, Peter May, J. E. McClure, M. Steinberger,
$H_\infty$ ring spectra and their applications, Lecture Notes in Math., Springer-Verlag, Berlin,
reviewed in (Ravenel 86, A1.5).
Further textbook accounts include
Harvey Margolis, Spectra and the Steenrod algebra, 1983 North-Holland
Stanley Kochmann, section 2.5 of Bordism, Stable Homotopy and Adams Spectral Sequences, AMS 1996
Stefan Schwede, chapter II section 10 of Symmetric spectra, 2012 (pdf)
Lecture notes include
Reviews include
(appendix 1, section 5 reviews the abstract algebraic definition).
On the dual Steenrod algebra:
For a commented list of furhter references see also
See also
The commutative Hopf algebroid structure on the dual $E$-Steenrod algebra $E_\bullet(E)$ and its relation to the $E^2$-term in the Adams spectral sequence is discussed in
Andrew Baker, Brave new Hopf algebroids, pdf
Andrew Baker, Andrey Lazarev, On the Adams Spectral Sequence for R-modules, Algebr. Geom. Topol. 1 (2001) 173-199 (arXiv:math/0105079)
Andrew Baker and Alain Jeanneret, Brave new Hopf algebroids and extensions of $MU$-algebras, Homology Homotopy Appl. Volume 4, Number 1 (2002), 163-173. (Euclid)
Mark Hovey, Homotopy theory of comodules over a Hopf algebroid (arXiv:math/0301229)
Last revised on April 21, 2019 at 10:27:18. See the history of this page for a list of all contributions to it.