Wednesday, 23 August 2006

examples - Request: A Serre fibration that is not a Dold fibration

You've already answered your own question, but here is another example.



Let f:mathbbQdeltatomathbbQ be the obvious map from the rational numbers with the discrete topology to the rational numbers with the usual topology. Let Mf be the mapping cylinder. Then the projection
p:Mfto[0,1]
is a Serre fibration. This follows because any map of a disc into mathbbQ factors through f. Hence as far as discs are concerned, Mf might as well be mathbbQdeltatimes[0,1].



However this projection is not a Dold fibration. It is easy to construct a diagram using Y=mathbbQ which will have no weak homotopy lift. Indeed consider the projection map mathbbQtimes[0,1]to[0,1] with the obvious initial lift. Any other initial lift vertically homotopic to this one in fact coincides with this one, hence it is easy to see that there is no weak lift of this map.



By replacing mathbbQdelta and mathbbQ with their cones, we get a similar example where now the base, total space, and fibers are contractible. Hence they are path-connected and also have the homotopy type of CW complexes. So this also answers Ronnie Browns question (but surely the answer to that has been known for some time).

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