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DOI: 10.4230/LIPIcs.ITP.2025.4

A Formalization of Divided Powers in Lean

Authors: Antoine Chambert-Loir and María Inés de Frutos-Fernández

Published in: LIPIcs, Volume 352, 16th International Conference on Interactive Theorem Proving (ITP 2025)

Abstract

Given an ideal I in a commutative ring A, a divided power structure on I is a collection of maps {γ_n : I → A}_{n ∈ ℕ}, subject to axioms that imply that it behaves like the family {x ↦ xⁿ/n!}_{n ∈ ℕ}, but which can be defined even when division by factorials is not possible in A. Divided power structures have important applications in diverse areas of mathematics, including algebraic topology, number theory and algebraic geometry. In this article we describe a formalization in Lean 4 of the basic theory of divided power structures, including divided power morphisms and sub-divided power ideals, and we provide several fundamental constructions, in particular quotients and sums. This constitutes the first formalization of this theory in any theorem prover. As a prerequisite of general interest, we expand the formalized theory of multivariate power series rings, endowing them with a topology and defining evaluation and substitution of power series.

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Antoine Chambert-Loir and María Inés de Frutos-Fernández. A Formalization of Divided Powers in Lean. In 16th International Conference on Interactive Theorem Proving (ITP 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 352, pp. 4:1-4:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{chambertloir_et_al:LIPIcs.ITP.2025.4,
  author =	{Chambert-Loir, Antoine and de Frutos-Fern\'{a}ndez, Mar{\'\i}a In\'{e}s},
  title =	{{A Formalization of Divided Powers in Lean}},
  booktitle =	{16th International Conference on Interactive Theorem Proving (ITP 2025)},
  pages =	{4:1--4:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-396-6},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{352},
  editor =	{Forster, Yannick and Keller, Chantal},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITP.2025.4},
  URN =		{urn:nbn:de:0030-drops-246038},
  doi =		{10.4230/LIPIcs.ITP.2025.4},
  annote =	{Keywords: Formal mathematics, algebraic number theory, commutative algebra, divided powers, Lean, Mathlib}
}

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DOI: 10.4230/LIPIcs.ITP.2023.13

Formalizing Norm Extensions and Applications to Number Theory

Authors: María Inés de Frutos-Fernández

Published in: LIPIcs, Volume 268, 14th International Conference on Interactive Theorem Proving (ITP 2023)

Abstract

The field ℝ of real numbers is obtained from the rational numbers ℚ by taking the completion with respect to the usual absolute value. We then define the complex numbers ℂ as an algebraic closure of ℝ. The p-adic analogue of the real numbers is the field ℚ_p of p-adic numbers, obtained by completing ℚ with respect to the p-adic norm. In this paper, we formalize in Lean 3 the definition of the p-adic analogue of the complex numbers, which is the field ℂ_p of p-adic complex numbers, a field extension of ℚ_p which is both algebraically closed and complete with respect to the extension of the p-adic norm. More generally, given a field K complete with respect to a nonarchimedean real-valued norm, and an algebraic field extension L/K, we show that there is a unique norm on L extending the given norm on K, with an explicit description. Building on the definition of ℂ_p, we formalize the definition of the Fontaine period ring B_{HT} and discuss some applications to the theory of Galois representations and to p-adic Hodge theory. The results formalized in this paper are a prerequisite to formalize Local Class Field Theory, which is a fundamental ingredient of the proof of Fermat’s Last Theorem.

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María Inés de Frutos-Fernández. Formalizing Norm Extensions and Applications to Number Theory. In 14th International Conference on Interactive Theorem Proving (ITP 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 268, pp. 13:1-13:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{defrutosfernandez:LIPIcs.ITP.2023.13,
  author =	{de Frutos-Fern\'{a}ndez, Mar{\'\i}a In\'{e}s},
  title =	{{Formalizing Norm Extensions and Applications to Number Theory}},
  booktitle =	{14th International Conference on Interactive Theorem Proving (ITP 2023)},
  pages =	{13:1--13:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-284-6},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{268},
  editor =	{Naumowicz, Adam and Thiemann, Ren\'{e}},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITP.2023.13},
  URN =		{urn:nbn:de:0030-drops-183880},
  doi =		{10.4230/LIPIcs.ITP.2023.13},
  annote =	{Keywords: formal mathematics, Lean, mathlib, algebraic number theory, p-adic analysis, Galois representations, p-adic Hodge theory}
}

Document

DOI: 10.4230/LIPIcs.ITP.2022.14

Formalizing the Ring of Adèles of a Global Field

Authors: María Inés de Frutos-Fernández

Published in: LIPIcs, Volume 237, 13th International Conference on Interactive Theorem Proving (ITP 2022)

Abstract

The ring of adèles of a global field and its group of units, the group of idèles, are fundamental objects in modern number theory. We discuss a formalization of their definitions in the Lean 3 theorem prover. As a prerequisite, we formalize adic valuations on Dedekind domains. We present some applications, including the statement of the main theorem of global class field theory and a proof that the ideal class group of a number field is isomorphic to an explicit quotient of its idèle class group.

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María Inés de Frutos-Fernández. Formalizing the Ring of Adèles of a Global Field. In 13th International Conference on Interactive Theorem Proving (ITP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 237, pp. 14:1-14:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{defrutosfernandez:LIPIcs.ITP.2022.14,
  author =	{de Frutos-Fern\'{a}ndez, Mar{\'\i}a In\'{e}s},
  title =	{{Formalizing the Ring of Ad\`{e}les of a Global Field}},
  booktitle =	{13th International Conference on Interactive Theorem Proving (ITP 2022)},
  pages =	{14:1--14:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-252-5},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{237},
  editor =	{Andronick, June and de Moura, Leonardo},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITP.2022.14},
  URN =		{urn:nbn:de:0030-drops-167232},
  doi =		{10.4230/LIPIcs.ITP.2022.14},
  annote =	{Keywords: formal math, algebraic number theory, class field theory, Lean, mathlib}
}

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Documents authored by de Frutos-Fernández, María Inés

A Formalization of Divided Powers in Lean

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Formalizing Norm Extensions and Applications to Number Theory

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Formalizing the Ring of Adèles of a Global Field

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