# Landweber exact functor theorem

In mathematics, the Landweber exact functor theorem, named after Peter Landweber, is a theorem in algebraic topology. It is known that a complex orientation of a homology theory leads to a formal group law. The Landweber exact functor theorem (or LEFT for short) can be seen as a method to reverse this process: it constructs a homology theory out of a formal group law.

## Statement

The coefficient ring of complex cobordism is $MU_{*}(*)=MU_{*}\cong \mathbb {Z} [x_{1},x_{2},\dots ]$, where the degree of $x_{i}$ is $2i$. This is isomorphic to the graded Lazard ring ${\mathcal}L_{*}$. This means that giving a formal group law F (of degree $-2$) over a graded ring $R_{*}$ is equivalent to giving a graded ring morphism $L_{*}\to R_{*}$. Multiplication by an integer $n>0$ is defined inductively as a power series, by
    $[n+1]^{F}x=F(x,[n]^{F}x)$ and $[1]^{F}x=x.$
Let now F be a formal group law over a ring ${\mathcal}R_{*}$. Define for a topological space X
    $E_{*}(X)=MU_{*}(X)\otimes _{MU_{*}}R_{*}$
Here $R_{*}$ gets its $MU_{*}$-algebra structure via F. The question is: is E a homology theory? It is obviously a homotopy invariant functor, which fulfills excision. The problem is that tensoring in general does not preserve exact sequences. One could demand that $R_{*}$ be flat over $MU_{*}$, but that would be too strong in practice. Peter Landweber found another criterion:
    Theorem (Landweber exact functor theorem)
    For every prime p, there are elements $v_{1},v_{2},\dots \in MU_{*}$ such that we have the following: Suppose that $M_{*}$ is a graded $MU_{*}$-module and the sequence $(p,v_{1},v_{2},\dots ,v_{n})$ is regular for $M$, for every p and n. Then
       $E_{*}(X)=MU_{*}(X)\otimes _{MU_{*}}M_{*}$
    is a homology theory on CW-complexes.
In particular, every formal group law F over a ring $R$ yields a module over ${\mathcal}MU_{*}$ since we get via F a ring morphism $MU_{*}\to R$.

## Remarks

* There is also a version for Brown–Peterson cohomology BP. The spectrum BP is a direct summand of $MU_{(p)}$ with coefficients $\mathbb {Z} _{(p)}[v_{1},v_{2},\dots ]$. The statement of the LEFT stays true if one fixes a prime p and substitutes BP for MU.
* The classical proof of the LEFT uses the Landweber–Morava invariant ideal theorem: the only prime ideals of $BP_{*}$ which are invariant under coaction of $BP_{*}BP$ are the $I_{n}=(p,v_{1},\dots ,v_{n})$. This allows to check flatness only against the $BP_{*}/I_{n}$ (see Landweber, 1976).
* The LEFT can be strengthened as follows: let ${\mathcal {E}}_{*}$ be the (homotopy) category of Landweber exact $MU_{*}$-modules and ${\mathcal {E}}$ the category of MU-module spectra M such that $\pi _{*}M$ is Landweber exact. Then the functor $\pi _{*}\colon {\mathcal {E}}\to {\mathcal {E}}_{*}$ is an equivalence of categories. The inverse functor (given by the LEFT) takes ${\mathcal}MU_{*}$-algebras to (homotopy) MU-algebra spectra (see Hovey, Strickland, 1999, Thm 2.7).

## Examples

The archetypical and first known (non-trivial) example is complex K-theory K. Complex K-theory is complex oriented and has as formal group law $x+y+xy$. The corresponding morphism $MU_{*}\to K_{*}$ is also known as the Todd genus. We have then an isomorphism
    $K_{*}(X)=MU_{*}(X)\otimes _{MU_{*}}K_{*},$
called the Conner–Floyd isomorphism.
While complex K-theory was constructed before by geometric means, many homology theories were first constructed via the Landweber exact functor theorem. This includes elliptic homology, the Johnson–Wilson theories $E(n)$ and the Lubin–Tate spectra $E_{n}$.
While homology with rational coefficients $H\mathbb {Q}$ is Landweber exact, homology with integer coefficients $H\mathbb {Z}$ is not Landweber exact. Furthermore, Morava K-theory K(n) is not Landweber exact.

## Modern reformulation

A module M over ${\mathcal}MU_{*}$ is the same as a quasi-coherent sheaf ${\mathcal {F}}$ over ${\text{Spec }}L$, where L is the Lazard ring. If $M={\mathcal}MU_{*}(X)$, then M has the extra datum of a ${\mathcal}MU_{*}MU$ coaction. A coaction on the ring level corresponds to that ${\mathcal {F}}$ is an equivariant sheaf with respect to an action of an affine group scheme G. It is a theorem of Quillen that $G\cong \mathbb {Z} [b_{1},b_{2},\dots ]$ and assigns to every ring R the group of power series
    $g(t)=t+b_{1}t^{2}+b_{2}t^{3}+\cdots \in R[[t]]$.
It acts on the set of formal group laws ${\text{Spec }}L(R)$ via
    $F(x,y)\mapsto gF(g^{-1}x,g^{-1}y)$.
These are just the coordinate changes of formal group laws. Therefore, one can identify the stack quotient ${\text{Spec }}L//G$ with the stack of (1-dimensional) formal groups ${\mathcal {M}}_{fg}$ and $M=MU_{*}(X)$ defines a quasi-coherent sheaf over this stack. Now it is quite easy to see that it suffices that M defines a quasi-coherent sheaf ${\mathcal {F}}$ which is flat over ${\mathcal {M}}_{fg}$ in order that $MU_{*}(X)\otimes _{MU_{*}}M$ is a homology theory. The Landweber exactness theorem can then be interpreted as a flatness criterion for ${\mathcal {M}}_{fg}$ (see Lurie 2010).

## Refinements to $E_{\infty }$-ring spectra

While the LEFT is known to produce (homotopy) ring spectra out of ${\mathcal}MU_{*}$, it is a much more delicate question to understand when these spectra are actually $E_{\infty }$-ring spectra. As of 2010, the best progress was made by Jacob Lurie. If X is an algebraic stack and $X\to {\mathcal {M}}_{fg}$ a flat map of stacks, the discussion above shows that we get a presheaf of (homotopy) ring spectra on X. If this map factors over $M_{p}(n)$ (the stack of 1-dimensional p-divisible groups of height n) and the map $X\to M_{p}(n)$ is etale, then this presheaf can be refined to a sheaf of $E_{\infty }$-ring spectra (see Goerss). This theorem is important for the construction of topological modular forms.