Quillen's theorems A and B

Quillen's Theorem A — If ${\displaystyle f:C\to D}$ is a functor such that the classifying space ${\displaystyle B(d\downarrow f)}$ of the comma category ${\displaystyle d\downarrow f}$ is contractible for any object d in D, then f induces a homotopy equivalence ${\displaystyle BC\to BD}$.

Quillen's Theorem B — If ${\displaystyle f:C\to D}$ is a functor that induces a homotopy equivalence ${\displaystyle B(d'\downarrow f)\to B(d\downarrow f)}$ for any morphism ${\displaystyle d\to d'}$ in D, then there is an induced long exact sequence:

${\displaystyle \cdots \to \pi _{i+1}BD\to \pi _{i}B(d\downarrow f)\to \pi _{i}BC\to \pi _{i}BD\to \cdots .}$