## Algebraic Topology: Lecture 13

We discussed the following two results: A Quillen adjunction (F, G, a)
between two model categories C and D gives rise to an adjunction (LF,
RG, Ra) between the homotopy categories Ho(C) and Ho(D). We call this
the *derived adjunction*. The Quillen adjunction (F, G, a)
is called a *Quillen equivalence* if for every cofibrant
object X of C and every fibrant object Y of D, a map f : F(X) --> Y
is a weak equivalence in D if and only if the adjoint map
a(f) : X --> G(Y) is a weak equivalence in C. In this case the derived
adjunction (LF, RG, Ra) is an *adjoint equivalence* between the
categories Ho(C) and Ho(D). This means that a map f : (LF)(X) --> Y
is an isomorphism in Ho(D) if and only if (Ra)(f) : X --> (RG)(Y) is
an isomorphism in Ho(C).
We then began a discussion of simplicial sets and of the adjunction
(| - |, Sin, a) between the categories of simplicial sets and
topological spaces given by the geometric realization functor and the
singular set functor. We mentioned that there exists a model structure
on the category of simplicial sets such that this adjunction is a
Quillen equivalence. Hence, for the purpose of doing homotopy theory,
there is no loss of information by considering simplicial sets instead
of topological spaces.