Approximate Fiber Products of Schemes and Their Étale Homotopical Invariants

Dongfang Zhao ()
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Dongfang Zhao: Tacoma School of Engineering & Technology, University of Washington, Seattle, WA 98195, USA

Mathematics, 2025, vol. 13, issue 21, 1-17

Abstract: The classical fiber product in algebraic geometry provides a powerful tool for studying loci where two morphisms to a base scheme, ϕ : X → S and ψ : Y → S , coincide exactly. This condition of strict equality, however, is insufficient for describing many real-world applications, such as the geometric structure of semantic spaces in modern large language models whose foundational architecture is the Transformer neural network: The token spaces of these models are fundamentally approximate, and recent work has revealed complex geometric singularities, challenging the classical manifold hypothesis. This paper develops a new framework to study and quantify the nature of approximate alignment between morphisms in the context of arithmetic geometry, using the tools of étale homotopy theory. We introduce the central object of our work, the étale mismatch torsor, which is a sheaf of torsors over the product scheme X × S Y . The structure of this sheaf serves as a rich, intrinsic, and purely algebraic object amenable to both qualitative classification and quantitative analysis of the global relationship between the two morphisms. Our main results are twofold. First, we provide a complete classification of these structures, establishing a bijection between their isomorphism classes and the first étale cohomology group H é t 1 ( X × S Y , π 1 é t ( S ) ̲ ) . Second, we construct a canonical filtration on this classifying cohomology group based on the theory of infinitesimal neighborhoods. This filtration induces a new invariant, which we term the order of mismatch, providing a hierarchical, algebraic measure for the degree of approximation between the morphisms. We apply this framework to the concrete case of generalized Howe curves over finite fields, demonstrating how both the characteristic class and its order reveal subtle arithmetic properties.

Keywords: approximate fiber product; étale topology; arithmetic geometry (search for similar items in EconPapers)
JEL-codes: C (search for similar items in EconPapers)
Date: 2025
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