On the computation of Kähler differentials and characterizations of Galois extensions with independent defect

Steven Dale Cutkosky, Franz‐Viktor Kuhlmann and Anna Rzepka

Mathematische Nachrichten, 2025, vol. 298, issue 5, 1549-1577

Abstract: For important cases of algebraic extensions of valued fields, we develop presentations of the associated Kähler differentials of the extensions of their valuation rings. We compute their annihilators as well as the associated differents. We then apply the results to Galois defect extensions of prime degree. Defects can appear in finite extensions of valued fields of positive residue characteristic and are serious obstructions to several problems in positive characteristic. A classification of defects (dependent vs. independent) has been introduced by the second and the third author. It has been shown that perfectoid fields and deeply ramified fields only admit extensions with independent defect. We give several characterizations of independent defect, using ramification ideals, Kähler differentials, and traces of the maximal ideals of valuation rings. All of our results are for arbitrary valuations; in particular, we have no restrictions on their ranks or value groups.

Date: 2025
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https://doi.org/10.1002/mana.202300532

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