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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.

The coefficient ring of complex cobordism is , where the degree of is . This is isomorphic to the graded Lazard ring . This means that giving a formal group law F (of degree ) over a graded ring is equivalent to giving a graded ring morphism . Multiplication by an integer is defined inductively as a power series, by

- and

Let now F be a formal group law over a ring . Define for a topological space *X*

Here gets its -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 be flat over , 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 such that we have the following: Suppose that is a graded -module and the sequence is regular for , for every
*p*and*n*. Then - is a homology theory on CW-complexes.

In particular, every formal group law F over a ring yields a module over since we get via F a ring morphism .

- There is also a version for Brown–Peterson cohomology BP. The spectrum BP is a direct summand of with coefficients . 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 which are invariant under coaction of are the . This allows to check flatness only against the (see Landweber, 1976).
- The LEFT can be strengthened as follows: let be the (homotopy) category of Landweber exact -modules and the category of MU-module spectra M such that is Landweber exact. Then the functor is an equivalence of categories. The inverse functor (given by the LEFT) takes -algebras to (homotopy) MU-algebra spectra (see Hovey, Strickland, 1999, Thm 2.7).

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 . The corresponding morphism is also known as the Todd genus. We have then an isomorphism

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 and the Lubin–Tate spectra .

While homology with rational coefficients is Landweber exact, homology with integer coefficients is not Landweber exact. Furthermore, Morava K-theory K(n) is not Landweber exact.

A module M over is the same as a quasi-coherent sheaf over , where L is the Lazard ring. If , then M has the extra datum of a coaction. A coaction on the ring level corresponds to that is an equivariant sheaf with respect to an action of an affine group scheme G. It is a theorem of Quillen that and assigns to every ring R the group of power series

- .

It acts on the set of formal group laws via

- .

These are just the coordinate changes of formal group laws. Therefore, one can identify the stack quotient with the *stack of (1-dimensional) formal groups* and 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 which is flat over in order that is a homology theory. The Landweber exactness theorem can then be interpreted as a flatness criterion for (see Lurie 2010).

While the LEFT is known to produce (homotopy) ring spectra out of , it is a much more delicate question to understand when these spectra are actually -ring spectra. As of 2010, the best progress was made by Jacob Lurie. If X is an algebraic stack and 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 (the stack of 1-dimensional p-divisible groups of height n) and the map is etale, then this presheaf can be refined to a sheaf of -ring spectra (see Goerss). This theorem is important for the construction of topological modular forms.

- Goerss, Paul. "Realizing families of Landweber exact homology theories" (PDF).
- Hovey, Mark; Strickland, Neil P. (1999), "Morava K-theories and localisation",
*Memoirs of the American Mathematical Society*,**139**(666), doi:10.1090/memo/0666, MR 1601906, archived from the original on 2004-12-07 - Landweber, Peter S. (1976). "Homological properties of comodules over and ".
*American Journal of Mathematics*.**98**(3): 591–610. doi:10.2307/2373808. JSTOR 2373808.. - Lurie, Jacob (2010). "Chromatic Homotopy Theory. Lecture Notes".