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SIMBa: ¿How far is an extension of $p$-adic fields from having a normal integral basis?


El proper dimecres, 10 de març, se celebrarà una nova xerrada —en format virtual— del Seminari Informal de Matemàtiques de Barcelona (SIMBa).

Speaker: Fabio Ferri
Universitat: University of Exeter

Data: Wednesday, March 10th, 2021.
Hora: 12:00,virtual coffee break; 12:20, talk.
Lloc: Zoom (the link will be posted on our website).
Idioma: English.

Títol: ¿How far is an extension of p-adic fields from having a normal integral basis?
Resum: Let L / K be a Galois extension of p -adic fields with Galois group G. Denote by K[G] the group ring \left\{\sum_{g \in G} a_{g} g: a_{g} \in K\right\} ; the classical normal basis theorem shows that L is a free K[G] -module of rank 1 . that is, there exists an element \alpha \in L such that \{g(\alpha)\}_{g \in G} is a basis of L as a K -vector space. It is natural to ask whether \mathcal{O}_{L} is also a free \mathcal{O}_{K}[G] -module of rank 1 , where \mathcal{O}_{L} and \mathcal{O}_{K} denote the rings of integers of L and K, respectively. A theorem of Noether tells us that this is the case if and only if the extension is (at most) tamely ramified. When L / K is wildly ramified, we can still note that there always exists a free \mathcal{O}_{K}[G] -submodule of \mathcal{O}_{L} with finite index. The purpose of this talk is to study the minimal such index, i.e. the quantity m(L / K):= \min _{\alpha \in \mathcal{O}_{L}}\left[\mathcal{O}_{L}: \mathcal{O}_{K}[G] \alpha\right] . We will provide a general bound that only depends on the invariants of the extension, a complete formula for m(L / K) when L / \mathbb{Q}_{p} is abelian and a complete formula when L / K is cyclic of degree p. This is joint work with Ilaria Del Corso and Davide Lombardo.


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