# Stiefel-Whitney classes of tensor product $\xi^m \otimes \eta^n$, computation

Let $\xi^m$ and $\eta^n$ be vector bundles over a paracompact base space. Where can I find a reference to the Stiefel-Whitney classes of the tensor product $\xi^m \otimes \eta^n$ being computed as follows?

There is a universal formula of the form$$w(\xi^m \otimes \eta^n) = p_{m, n} (w_1(\xi^m), \dots, w_m(\xi^m), w_1(\eta^n), \dots, w_n(\eta^n)),$$where the polynomial $p_{m, n}$ in $m + n$ variables can be characterized as follows. If $\sigma_1, \dots, \sigma_m$ are the elementary symmetric functions of indeterminates $t_1, \dots, t_m$, and if $\sigma_1', \dots, \sigma_n'$ are the elementary symmetric functions of $t_1', \dots, t_n'$, then$$p_{m, n}(\sigma_1, \dots, \sigma_m, \sigma_1', \dots, \sigma_n') = \prod_{i = 1}^n \prod_{j = 1}^n (1 + t_i + t_j').$$

The keyword is "splitting principle". The problem itself is a copy of an exercise from $\S7$ in Milnor-Stasheff, which has a nice hint.

For the detailed proof see Proposition 3.2.12 in this notes http://www.analg.ulg.ac.be/jps/rec/icc.pdf

I'm not sure you need the splitting principle (which isn't mentioned in the book by Milnor and Stasheff): if you prove the formula for canonical bundles $\gamma^m,\gamma^n$ over the grassmanianns $G_m, G_n$, you can use the existence of $\iota_m:\xi^m\to\gamma^m$ and $\iota_n:\zeta^n\to\gamma^n$ to deduce the formula for $\xi^m,\zeta^n$ (no need for $\iota^*$ to be injective in that sense !)

So $w(\gamma^m\otimes\gamma^n)\in H^*(G_m\times G_n)$, which Künneth's theorem computes from $H^*(G_m)$ and $H^*(G_n)$, showing us that $w(\gamma^m\otimes\gamma^n)$ is some polynomial in the Stieffel-Whitney classes of $G_n$ and $G_m$.

To express this polynomial, we split $G_n$ and $G_m$ over the cartesian products of $n$ (respectively $m$) infinite dimensional projective spaces (let us denote by $\iota_n,\iota_m$ the morphisms from these products to the grassmannians) [Milnor, Stasheff] proves that the corresponding $\iota^*$ are injective and that the SW classes of $\iota^*(w_k(\gamma^n))$ is the elementary symmetric polynomial of degree $k$ in $a_1,\ldots,a_n$ where $a_i$ is the SW class of the canonical line bundle in the $i$_th copy of $P^{\infty}$.

This is for our right hand-side in Künneth's theorem. What happens to $w(\gamma^m\otimes\gamma^n)\in H^*(G_m\times G_n)$ when we split? It becomes $w((\gamma^1\times\ldots\times\gamma^1)\otimes(\gamma^1\times\ldots\times\gamma^1))$ with $m$ copies first, $n$ copies then. Just distribute, use the formula for the SW class of a cartesian product and the fact that $w_1(\gamma^1\otimes\gamma^1)=a_i+b_j$ where $a_i,b_j$ are the $w_1$ of each copy of $\gamma^1$ to conclude.

• but that is the splitting principle... – Dylan Wilson Mar 14 '18 at 19:19