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Let $p:E\to B$ be a fibration. Suppose $B$ has a simplicial decomposition. For each $n\in\mathbb{Z}_{\ge0}$, let $C_n$ be the free abelian group generated by the set of pairs $(\sigma,\tau)$ where $\sigma:\Delta^n\to E$ is a singular simplex, $\tau:\Delta^n\to B$ is a simplicial simplex, and $\tau=p\circ\sigma$. There is a boundary operator $\partial:C_n\to C_{n-1}$ defined in the usual way, making use of the faces of $\Delta^n$. Clearly $\partial\circ\partial=0$, and $(C_*,\partial)$ is a chain complex.

Update: If $B$ is a finite-dimensional simplicial complex, then for $n>\dim B$, $C_n$ is defined inductively as follows. For $n=\dim B+1$, let $C_{\dim B+1}$ be the free abelian group generated by the set of pairs $(\sigma,\tau)$ where $\sigma:\Delta^n\to E$ is a singular simplex, $\tau:\Delta^n\to B$ is a singular simplex such that $\tau=p\circ\sigma$ and $\partial(\sigma,\tau)\in C_{\dim B}$, where $\partial(\sigma,\tau)$ is defined in the usual way. Define $C_n$ similarly for all larger $n$.

My question is: Is the homology of $(C_*,\partial)$ isomorphic to the singular homology of $E$?

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    $\begingroup$ I think I know how to prove it: your complex includes in the usual singular complex for E; filter both by the skeletal filtration on the base; an inductive proof shows that the inclusion induces an isomorphism on exact couples, which gives an isomorphism on spectral sequences; finally, noting that the larger filtered complex defines the Serre spectral sequence, we obtain the result. $\endgroup$ Nov 5, 2022 at 13:46
  • $\begingroup$ But I'm guessing that the proof of the Serre spectral sequence actually uses a result like this one, so I'm hoping you get an answer that is short and sweet! $\endgroup$ Nov 5, 2022 at 13:47
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    $\begingroup$ Cross-posted on MSE. $\endgroup$
    – Tyrone
    Nov 5, 2022 at 16:00

1 Answer 1

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My question is: Is the homology of $(C_*,\partial)$ isomorphic to the singular homology of E?

Yes. Observe that the singular complex functor sends Serre fibrations to Kan fibrations. Thus, the map $$\def\Sing{\mathop{\rm Sing}} \Sing p\colon \Sing E\to \Sing B$$ is a Kan fibration.

Denote by $T$ the underlying simplicial set of the triangulation of $B$ so that we have a homeomorphism $|T|→B$ and an adjoint simplicial weak equivalence $t\colon T→\Sing B$.

Now the chain complex $C$ is simply the simplicial chains of the pullback of the span of simplicial sets $$T→\Sing B←\Sing E.$$

The base change of $\Sing p$ along $t$ yields a map $P→T$.

One leg of the above span is a Kan fibration, therefore the pullback computes the homotopy pullback. The map $T→\Sing B$ is a weak equivalence, hence so is its base change $P\to \Sing E$.

Thus, the map $P→T$ is weakly equivalent to $\Sing p$, so the chain complex of $P$ is quasi-isomorphic to the chain complex of $\Sing E$. The chain complex of $P$ is precisely $C$.

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  • $\begingroup$ In case you aren't familiar with the construction, \DeclareMathOperator\Sing{Sing} has substantially the same result as \def\Sing{\mathop{\rm Sing}}. $\endgroup$
    – LSpice
    Nov 5, 2022 at 15:40
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    $\begingroup$ @LSpice: If it has the same result, then what is your point? $\endgroup$ Nov 5, 2022 at 17:04
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    $\begingroup$ Re, increased knowledge, if you didn't already know, and hopefully no harm done, if you did already know. $\endgroup$
    – LSpice
    Nov 5, 2022 at 18:14

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