# Quillen'S Theorems A And B

### (Theorems In Topology)

Tech

###### Theorems In Topology ## What is Quillen's theorems A and B?

In topology, a branch of mathematics, Quillen's Theorem A gives a sufficient condition for the classifying spaces of two categories to be homotopy equivalent. Quillen's Theorem B gives a sufficient condition for a square consisting of classifying spaces of categories to be . The two theorems play central roles in Quillen's Q-construction in algebraic K-theory and are named after Daniel Quillen. The precise statements of the theorems are as follows. Quillen's Theorem A — If is a functor such that the classifying space of the comma category is contractible for any object d in D, then f induces a homotopy equivalence . Quillen's Theorem B — If is a functor that induces a homotopy equivalence for any morphism , then there is an induced long exact sequence: In general, the homotopy fiber of is not naturally the classifying space of a category: there is no natural category such that . Theorem B constructs in a case when is especially nice.

### Technology Types

theorems in topology

### Synonyms

Quillen's theoremQuillen's Theorem AQuillen's Theorem BQuillen’s theorem A

## Tech Info

Source: [object Object]
— Date merged: 11/6/2021, 1:32:46 PM
— Date scraped: 5/20/2021, 7:17:41 PM