English

The condensed homotopy type of a scheme

Algebraic Geometry 2025-10-10 v1 Algebraic Topology

Abstract

We study a condensed version of the \'etale homotopy type of a scheme, which refines both the usual \'etale homotopy type of Friedlander-Artin-Mazur and the pro\'etale fundamental group of Bhatt-Scholze. In the first part of this paper, we prove that this condensed homotopy type satisfies descent along integral morphisms and that the expected fiber sequences hold. We also provide explicit computations, for example, for rings of continuous functions. A key ingredient in many of our arguments is a description of the condensed homotopy type using the Galois category of a scheme introduced by Barwick-Glasman-Haine. In the second part, we focus on the fundamental group of the condensed homotopy type in more detail. We show that, unexpectedly, the fundamental group of the condensed homotopy type of the affine line AC1\mathbf{A}^1_{\mathbf{C}} over the complex numbers is nontrivial. Nonetheless, its Noohi completion recovers the pro\'etale fundamental group of Bhatt-Scholze. Moreover, we show that a mild correction, passing to the quasiseparated quotient, fixes most of this group's quirks. Surprisingly, this quotient is often a topological group.

Keywords

Cite

@article{arxiv.2510.07443,
  title  = {The condensed homotopy type of a scheme},
  author = {Peter J. Haine and Tim Holzschuh and Marcin Lara and Catrin Mair and Louis Martini and Sebastian Wolf with an appendix by Bogdan Zavyalov},
  journal= {arXiv preprint arXiv:2510.07443},
  year   = {2025}
}

Comments

Comments very welcome. 103 pages