Cartan formula
# Cartan formula

In mathematics, Cartan formula can mean:

    ${\begin{aligned}Sq^{n}(x\smile y)&=\sum _{i+j=n}(Sq^{i}x)\smile (Sq^{j}y)\quad {\text{or}}\\P^{n}(x\smile y)&=\sum _{i+j=n}(P^{i}x)\smile (P^{j}y)\end{aligned}}$.

See Steenrod algebra for the detail. The name derives from Henri Cartan, son of Élie.

## Footnotes

* ↑ More precisely, these five axioms define 'cohomology operations', which are natural transformations between cohomology functors, which in turn define Steenrod algebra.
