# An Approximate Nerve Theorem

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## Abstract

The nerve theorem relates the topological type of a suitably nice space with the nerve of a good cover of that space. It has many variants, such as to consider acyclic covers and numerous applications in topology including applied and computational topology. The goal of this paper is to relax the notion of a good cover to an approximately good cover, or more precisely, we introduce the notion of an \(\varepsilon \)-acyclic cover. We use persistent homology to make this rigorous and prove tight bounds between the persistent homology of a space endowed with a function and the persistent homology of the nerve of an \(\varepsilon \)-acyclic cover of the space. Our approximations are stated in terms of *interleaving distance* between persistence modules. Using the Mayer–Vietoris spectral sequence, we prove upper bounds on the interleaving distance between the persistence module of the underlying space and the persistence module of the nerve of the cover. To prove the best possible bound, we must introduce special cases of interleavings between persistence modules called left and right interleavings. Finally, we provide examples which achieve the bound proving the lower bound and tightness of the result.

## Keywords

Persistence modules Mayer–Vietoris Spectral sequences Approximation## Mathematics Subject Classification

Primary 55 55T 18## Notes

### Acknowledgements

The authors would like to thank Don Sheehy for introducing them to the problem and to Petar Pavešić for suggesting the proof of Proposition 4.7. The first author was supported by the Slovenian Research Agency Grant P1-0292-0101. The second author was supported by the Slovenian Research Agency Grant TopRep N1-0058.

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