Based vs unbased fibration: Fiber sequence

algebraic-topologyfibrationgeneral-topologyhomotopy-theory

My question is focused on basepoint issues in topology. The starting point is basically Problem 4.3.18 in Hatcher which asks us to show that a fibration sequence induces a long exact sequence after applying $\langle X, – \rangle$. I'm interested in:

Let $F \xrightarrow{i} E \xrightarrow{p} B$ be a fibration, i.e. $F = p^{-1}(b_0)$ is a fiber for a chosen basepoint $b_0 \in B$. Then, the sequence of based sets $\langle X, F \rangle \to \langle X, E \rangle \to \langle X, B \rangle$ is exact for any topological space $X$.

So I'm confused about the basepoint here. I believe Hatcher wants $p$ to be an unbased fibration (because based fibrations never appear in his book). But I wonder if this result is even true in this generality for unbased fibrations – it feels weird for me to switch between unbased and based categories.

If I want to show $\ker{p_*} \subseteq \operatorname{im}{i_*}$, then I want to lift a homotopy but my proof breaks down since this homotopy needs not be basepoint-preserving. It appears that the result is true for well-pointed spaces $X$ because $p$ satisfies the based homotopy lifting property with respect to well-pointed spaces (see e.g. Tyrone Cutler, Lemma 1.3). The same notes contain basically the same proof I'm suggesting for this result, only that everything is assumed to be in the based categories (Tyrone Cutler, Proposition 1.6).

Does Hatcher tacitly mean that $p$ is a based fibration or that $X$ is a nice space or is there some technique that can make this work even in this generality?

Best Answer

It is hard to say what Hatcher really means, unfortunately he isn't always precise in his formulations.

As far as I know we need a pointed fibration to get the eaxct sequence in your question. However, if you want to work with free fibrations $p : E \to B$, you can easily prove that for each well-pointed $(X,x_0)$ you get an exact sequence $$\langle (X,x_0), (F,e_0)\rangle \to \langle (X,x_0), (E,e_0)\rangle \to \langle (X,x_0), (B,b_0)\rangle$$

I guess this is what Hatcher has in mind since he mainly works with CW-complexes in Chapter 4 (he does not say anything explicitly about $X$).

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