Bivariant theory

Unlike a homology theory or a cohomology theory, a bivariant class is defined for a map not a space.

Let ${\displaystyle f:X\to Y}$ be a map. For such a map, we can consider the fiber square

${\displaystyle {\begin{matrix}X'&\to &Y'\\\downarrow &&\downarrow \\X&\to &Y\end{matrix}}}$

(for example, a blow-up.) Intuitively, the consideration of all the fiber squares like the above can be thought of as an approximation of the map ${\displaystyle f}$.

Now, a birational class of ${\displaystyle f}$ is a family of group homomorphisms indexed by the fiber squares:

${\displaystyle A_{k}Y'\to A_{k-p}X'}$

satisfying the certain compatibility conditions.

The basic question was whether there is a cycle map:

${\displaystyle A^{*}(X)\to \operatorname {H} ^{*}(X,\mathbb {Z} ).}$

If X is smooth, such a map exists since ${\displaystyle A^{*}(X)}$ is the usual Chow ring of X. (Totaro 2014) harv error: no target: CITEREFTotaro2014 (help) has shown that rationally there is no such a map with good properties even if X is a linear variety, roughly a variety admitting a cell decomposition. He also notes that Voevodsky's motivic cohomology ring is "probably more useful " than the operational Chow ring for a singular scheme (§ 8 of loc. cit.)

• Burt Totaro, Chow groups, Chow cohomology, and linear varieties, Forum Math. Sigma 2 (2014), e17, 25.
• Dan Edidin and Matthew Satriano, Towards an intersection Chow cohomology for GIT quotients
• Fulton, William (1998), Intersection Theory, Berlin, New York: Springer-Verlag, ISBN 978-0-387-98549-7, MR 1644323
• William Fulton, Robert MacPherson, Categorical framework for the study of singular spaces, Memoirs of the AMS, 243, 1981
• The last two lectures of Vakil, Math 245A Topics in algebraic geometry: Introduction to intersection theory in algebraic geometry

• https://ncatlab.org/nlab/show/bivariant+cohomology+theory