# Yoneda product

In algebra, the Yoneda product is the pairing between Ext groups of modules:

$\operatorname{Ext}^n(M, N) \otimes \operatorname{Ext}^m(L, M) \to \operatorname{Ext}^{n+m}(L, N)$

induced by

$\operatorname{Hom}(M, N) \otimes \operatorname{Hom}(L, M) \to \operatorname{Hom}(L, N),\, f \otimes g \mapsto f \circ g.$

Specifically, for an element $\xi \in \operatorname{Ext}^n(M, N)$, thought of as an extension

$\xi : 0 \rightarrow N \rightarrow E_0 \rightarrow \cdots \rightarrow E_{n-1} \rightarrow M \rightarrow 0$,

and similarly

$\rho : 0 \rightarrow M \rightarrow F_0\rightarrow \cdots \rightarrow F_{m-1} \rightarrow L \rightarrow 0 \in \operatorname{Ext}^m(L, M)$,

we form the Yoneda (cup) product

$\xi \smile \rho : 0 \rightarrow N \rightarrow E_0 \rightarrow \cdots \rightarrow E_{n-1} \rightarrow F_0 \rightarrow \cdots \rightarrow F_{m-1} \rightarrow L \rightarrow 0 \in \operatorname{Ext}^{m + n}(L, N)$.

Note that the middle map $E_{n-1} \rightarrow F_1$ factors through the given maps to $M$.

We extend this definition to include $m, n = 0$ using the usual functoriality of the $\operatorname{Ext}^*(\_,\_)$ groups.