Bousfield and Kan’s
–completion
and fiberwise
–completion
of spaces lead to two different approaches to the rational homotopy theory of nonsimply
connected spaces. In the first approach, a map is a weak equivalence if it induces an
isomorphism on rational homology. In the second, a map of path-connected
pointed spaces is a weak equivalence if it induces an isomorphism between
fundamental groups and higher rationalized homotopy groups; we call these maps
–rational
homotopy equivalences. We compare these two notions and show that
–rational
homotopy equivalences correspond to maps that induce
–quasi-isomorphisms
on the rational singular chains, ie maps that induce a quasi-isomorphism
after applying the cobar functor to the dg coassociative coalgebra of rational
singular chains. This implies that both notions of rational homotopy
equivalence can be deduced from the rational singular chains by using
different algebraic notions of weak equivalences: quasi-isomorphisms and
–quasi-isomorphisms.
We further show that, in the second approach, there are no dg coalgebra models of
the chains that are both strictly cocommutative and coassociative.
Keywords
rational homotopy theory, cobar construction, fundamental
group