Adams resolution

In mathematics, specifically algebraic topology, there is a resolution analogous to free resolutions of spectra yielding a tool for constructing the Adams spectral sequence. Essentially, the idea is to take a connective spectrum of finite type $X$ and iteratively resolve with other spectra that are in the homotopy kernel of a map resolving the cohomology classes in $H^{*}(X;\mathbb{Z}/p)$ using Eilenberg–MacLane spectra.

This construction can be generalized using a spectrum $E$, such as the Brown–Peterson spectrum $BP$, or the complex cobordism spectrum $MU$, and is used in the construction of the Adams–Novikov spectral sequence^{pg 49}.

Construction

The mod

$p$

Adams resolution

$(X_s,g_s)$

for a spectrum

$X$

is a certain "chain-complex" of spectra induced from recursively looking at the fibers of maps into generalized Eilenberg–Maclane spectra giving generators for the cohomology of resolved spectra^{pg 43}. By this, we start by considering the map

$\begin{array}{c}X\\ \downarrow \\ K\end{array}$

where

$K$

is an Eilenberg–Maclane spectrum representing the generators of

$H^{*}(X)$

, so it is of the form

$K=\underset{k=1}{\overset{\mathrm{\infty }}{\bigvee }}\underset{I_k}{\bigvee }{\mathrm{\Sigma }}^{k}H\mathbb{Z}/p$

where

$I_k$

indexes a basis of

$H^k(X)$

, and the map comes from the properties of Eilenberg–Maclane spectra. Then, we can take the homotopy fiber of this map (which acts as a homotopy kernel) to get a space

$X_1$

. Note, we now set

$X_0=X$

and

$K_0=K$

. Then, we can form a commutative diagram

$\begin{array}{ccc}{X}_0& \leftarrow & X_1\\ \downarrow & & \\ K_0\end{array}$

where the horizontal map is the fiber map. Recursively iterating through this construction yields a commutative diagram

$\begin{array}{cccccc}{X}_0& \leftarrow & X_1& \leftarrow & X_2& \leftarrow \cdots \\ \downarrow & & \downarrow & & \downarrow \\ K_0& & K_1& & K_2\end{array}$

giving the collection

$(X_s,g_s)$

. This means

$X_s=\text{Hofiber}(f_{s-1}:X_{s-1}\to K_{s-1})$

is the homotopy fiber of

$f_{s-1}$

and

$g_s:X_s\to X_{s-1}$

comes from the universal properties of the homotopy fiber.

Resolution of cohomology of a spectrum

Now, we can use the Adams resolution to construct a free

${𝒜}_p$

-resolution of the cohomology

$H^{*}(X)$

of a spectrum

$X$

. From the Adams resolution, there are short exact sequences

$0\leftarrow H^{*}(X_s)\leftarrow H^{*}(K_s)\leftarrow H^{*}(\mathrm{\Sigma }{X}_{s+1})\leftarrow 0$

which can be strung together to form a long exact sequence

$0\leftarrow H^{*}(X)\leftarrow H^{*}(K_0)\leftarrow H^{*}(\mathrm{\Sigma }{K}_1)\leftarrow H^{*}({\mathrm{\Sigma }}^2{K}_2)\leftarrow \cdots$

giving a free resolution of

$H^{*}(X)$

as an

${𝒜}_p$

-module.

E_*-Adams resolution

Because there are technical difficulties with studying the cohomology ring

$E^{*}(E)$

in general^{pg 280}, we restrict to the case of considering the homology coalgebra

$E_{*}(E)$

(of co-operations). Note for the case

$E=H{𝔽}_p$

,

$H{𝔽}_{p*}(H{𝔽}_p)={𝒜}_{*}$

is the dual Steenrod algebra. Since

$E_{*}(X)$

is an

$E_{*}(E)$

-comodule, we can form the bigraded group

${\text{Ext}}_{E_{*}(E)}(E_{*}(𝕊),E_{*}(X))$

which contains the

$E_2$

-page of the Adams–Novikov spectral sequence for

$X$

satisfying a list of technical conditions^{pg 50}. To get this page, we must construct the

$E_{*}$

-Adams resolution^{pg 49}, which is somewhat analogous to the cohomological resolution above. We say a diagram of the form

$\begin{array}{cccccc}{X}_0& \stackrel{g_0}{\leftarrow }& X_1& \stackrel{g_1}{\leftarrow }& X_2& \leftarrow \cdots \\ \downarrow & & \downarrow & & \downarrow \\ K_0& & K_1& & K_2\end{array}$

where the vertical arrows

$f_s:X_s\to K_s$

is an

$E_{*}$

-Adams resolution if

1. $X_{s+1}=\text{Hofiber}(f_s)$ is the homotopy fiber of $f_s$
2. $E\wedge X_s$ is a retract of $E\wedge K_s$, hence $E_{*}(f_s)$ is a monomorphism. By retract, we mean there is a map $h_s:E\wedge K_s\to E\wedge X_s$ such that $h_s(E\wedge f_s)=i{d}_{E\wedge X_s}$
3. $K_s$ is a retract of $E\wedge K_s$
4. ${\text{Ext}}^{t,u}(E_{*}(𝕊),E_{*}(K_s))={\pi }_u(K_s)$ if $t=0$, otherwise it is $0$

Although this seems like a long laundry list of properties, they are very important in the construction of the spectral sequence. In addition, the retract properties affect the structure of construction of the $E_{*}$-Adams resolution since we no longer need to take a wedge sum of spectra for every generator.

Construction for ring spectra

The construction of the

$E_{*}$

-Adams resolution is rather simple to state in comparison to the previous resolution for any associative, commutative, connective ring spectrum

$E$

satisfying some additional hypotheses. These include

$E_{*}(E)$

being flat over

${\pi }_{*}(E)$

,

${\mu }_{*}$

on

${\pi }_0$

being an isomorphism, and

$H_r(E;A)$

with

$\mathbb{Z}\subset A\subset \mathbb{Q}$

being finitely generated for which the unique ring map

$\theta :\mathbb{Z}\to {\pi }_0(E)$

extends maximally. If we set

$K_s=E\wedge F_s$

and let

$f_s:X_s\to K_s$

be the canonical map, we can set

$X_{s+1}=\text{Hofiber}(f_s)$

Note that

$E$

is a retract of

$E\wedge E$

from its ring spectrum structure, hence

$E\wedge X_s$

is a retract of

$E\wedge K_s=E\wedge E\wedge X_s$

, and similarly,

$K_s$

is a retract of

$E\wedge K_s$

. In addition

$E_{*}(K_s)=E_{*}(E){\otimes }_{{\pi }_{*}(E)}{E}_{*}(X_s)$

which gives the desired

$\text{Ext}$

terms from the flatness.

Relation to cobar complex

It turns out the $E_1$-term of the associated Adams–Novikov spectral sequence is then cobar complex $C^{*}(E_{*}(X))$.

See also

• Adams spectral sequence
• Adams–Novikov spectral sequence
• Eilenberg–Maclane spectrum
• Hopf algebroid