Cap product

In algebraic topology the cap product is a method of adjoining a chain of degree ${\displaystyle p}$ with a cochain of degree ${\displaystyle q}$, such that ${\displaystyle q\leq p}$, to form a composite chain of degree ${\displaystyle p-q}$. It was introduced by Eduard Čech in 1936, and independently by Hassler Whitney in 1938.