Subject: categories: Re: name for a concept

From: "Clemens.BERGER" <[email-masked]>
Date: Thu, 08 Dec 2005 12:26:08 +0100

In my previous message, one should read:

a commutative square with two parallel fibrations in a right proper
model category is a homotopy pullback if and only if the square is exact
with respect to the class of weak equivalences. These special squares
compose because weak equivalences are stable under base change along
fibrations (this is the definition of right proper).

Clemens Berger.