nLab
nerve theorem

This page is about a property of Čech nerves in homotopy theory. For the “nerve theorem” in category theory see at Segal conditions.

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Idea

The nerve theorem asserts that the homotopy type of a sufficiently nice topological space is encoded in the Cech nerve of a good cover.

This can be seen as a special case of some aspects of étale homotopy as the étale homotopy type of nice spaces coincides with the homotopy type of its Cech nerve.

Statement

Theorem

Let $X$ be a paracompact space and $\{U_i\to X\}$ a good open cover. Write $C(\{U_i\})$ for the Cech nerve of this cover

$$C(\{U_i\})=(\overrightarrow{\overrightarrow{\to }}\coprod _{i,j}{U}_i\cap U_j\overrightarrow{\to }\coprod _i{U}_i),$$C(\{U_i\}) = \left( \stackrel{\to}{\stackrel{\to}{\to}}\coprod_{i,j} U_i \cap U_j \stackrel{\to}{\to} \coprod_{i} U_i \right) \,,

(a simplicial space) and write

$$\tilde{C}(\{U_i\})=(\overrightarrow{\overrightarrow{\to }}\coprod _{i,j}*\overrightarrow{\to }\coprod _i*)$$\tilde C(\{U_i\}) = \left( \stackrel{\to}{\stackrel{\to}{\to}}\coprod_{i,j} * \stackrel{\to}{\to} \coprod_{i} * \right)

for the simplicial set obtained by replacing in $C(\{U_i\})$ each direct summand space by the point. Let $\mid \tilde{C}(\{U_i\})\mid$ be the geometric realization.

This is homotopy equivalent to $X$.

This is usually attributed to (Borsuk1948). The proof relies on the existence of partitions of unity (see for instance the review Hatcher, prop. 4G.2).

Remark

This statement implies that in the cohesive (∞,1)-topos ETop∞Grpd the intrinsic fundamental ∞-groupoid in a locally ∞-connected (∞,1)-topos coincides with the ordinary fundamental ∞-groupoid functor of paracompact topological spaces. See Euclidean-topological ∞-groupoid : Geometric homotopy for details.

References

The nerve theorem is usually attributed to

• K. Borsuk, On the imbedding of systems of compacta in simplicial complexes , Fund. Math 35, (1948) 217-234

A review appears as corollary 4G.3 in the textbook

• Allen Hatcher, Algebraic topology (web) .

Some slightly stronger statements are discussed in

• Anders Björner, Nerves, fibers and homotopy groups , Journal of combinatorial theory, series A, 102 (2003), 88-93

• Andrzej Nagórko, Carrier and nerve theorems in the extension theory Proc. Amer. Math. Soc. 135 (2007), 551-558. (web)