cofibration category


Model category theory

model category



Universal constructions


Producing new model structures

Presentation of (,1)(\infty,1)-categories

Model structures

for \infty-groupoids

for ∞-groupoids

for nn-groupoids

for \infty-groups

for \infty-algebras



for stable/spectrum objects

for (,1)(\infty,1)-categories

for stable (,1)(\infty,1)-categories

for (,1)(\infty,1)-operads

for (n,r)(n,r)-categories

for (,1)(\infty,1)-sheaves / \infty-stacks

Homotopy theory

homotopy theory, (∞,1)-category theory, homotopy type theory

flavors: stable, equivariant, rational, p-adic, proper, geometric, cohesive, directed

models: topological, simplicial, localic, …

see also algebraic topology



Paths and cylinders

Homotopy groups

Basic facts




A cofibration category is a category equipped with a sub-class of its morphisms which behave like cofibrations.

The notion of cofibration category was introduced by Hans-Joachim Baues (see references below) as a variant of the category of cofibrant objects, (for which, see category of fibrant objects and dualise). The axioms are substantially weaker than those of a Quillen model category, but add one axiom to those of K. S. Brown.

In the first chapter of his book Algebraic Homotopy (see below), Baues suggests two criteria for an axiom system:

  1. The axioms should be sufficiently strong to permit the basic constructions of homotopy theory.

  2. The axioms should be as weak (and as simple) as possible, so that the constructions of homotopy theory are available in as many contexts as possible.

Baues also introduces the notion of a I-category being a category with a natural cylinder functor satisfying a set of good properties.

One should distinguish cofibration categories from the Waldhausen’s notion of a “category with cofibrations and weak equivalences”, nowdays called Waldhausen category.


A cofibration category is a category CC with two classes of morphisms, cof\mathit{cof} of cofibrations and w.e.w.e. of weak equivalences. These are to satisfy:

C 1) Isomorphisms are both cofibrations and weak equivalences. If

AfBgCA\stackrel{f}{\rightarrow} B \stackrel{g}{\rightarrow} C

are composable morphisms in CC, then if two of ff, gg, gfg \circ f are in w.e.w.e., so is the third; if ff, gcofg \in \mathit{cof} then gfcofg \circ f\in \mathit{cof}.

C2) Pushout axiom:

Given BAB\rightarrow A in cof\mathit{cof} and any f:BYf : B\rightarrow Y, the pushout

B f Y i i¯ A f¯ A BY \array{ B &\stackrel{f}{\rightarrow}& Y \\ \downarrow_i && \downarrow^{\bar{i}} \\ A &\stackrel{\bar{f}}{\to}& A \cup_B Y }

exists and i¯\bar{i} is in cof\mathit{cof}; if ff is in w.e.w.e., so is f¯\bar{f}; if ii is also in w.e.w.e., so is i¯\bar{i}.

C3) Factorisation axiom:

Given BfYB\stackrel{f}{\rightarrow} Y, there is a factorisation

B f Y i g A \array{ B&\stackrel{f}{\longrightarrow}&Y\\ \stackrel{i}{\searrow}&&\stackrel{g}{\nearrow}\\ &A& }

with icofi \in \mathit{cof}, gw.e.g\in w.e.

C4) Axiom on fibrant models:

Using the terminology: “ff is a trivial cofibration” to mean “fcofw.e.f\in \mathit{cof} \cap w.e.”, and “RXRX is fibrant” to mean “Given any trivial cofibration i:RXQi: RX \stackrel{\simeq}{\rightarrow}Q, there is a retraction r:QRXr : Q \rightarrow RX, ri=Id RXr \circ i = Id_{RX}”, the axiom states:

Given XCX\in C, there is trivial cofibration XRXX\rightarrow RX with RXRX fibrant.


  • If CC has a model category structure then the full subcategory of cofibrant objects forms a cofibration category.


  • H.J. Baues, Algebraic homotopy, Cambridge studies in advanced mathematics 15, Cambridge University Press, (1989).

Revised on May 28, 2017 09:03:38 by Urs Schreiber (