Abstract
Let K be a number field of degree n over \(\mathbb {Q}\). Then the 4-rank of the strict class group of K is at least \(\text {rank}_2 \, ( E_{K}^{+} / E_K^2) - \lfloor n /2 \rfloor \) where \(E_K\) and \( E_{K}^{+} \) denote the units and the totally positive units of K, respectively, and \(\text {rank}_2\) is the dimension as an elementary abelian 2-group. In particular, the strict class group of a totally real field K with a totally positive system of fundamental units contains at least \((n-1)/2\) (n odd) or \(n/2 -1\) (n even) independent elements of order 4. We also investigate when units in K are sums of two squares in K or are squares mod 4 in K.
Résumé
Soit K un corps de nombres de degré n sur \(\mathbb {Q}\). Alors le 4-rang du groupe de classes au sens strict de K est au moins \(\text {rang}_2 ( E_{K}^{+} / E_K^2) - \lfloor n /2 \rfloor \) où \(E_K\) et \( E_{K}^{+} \) désignent respectivement le groupe des unités et le groupe des unités totalement positives de K, et où \(\text {rang}_2\) est la dimension en tant que 2-groupe abélien élémentaire. En particulier, le groupe des classes au sens strict d’un corps K totalement réel avec un système fondamental d’unités totalement positives contient au moins \((n-1)/2\) pour n impair (respectivement \(n/2 -1\) pour n pair) éléments indépendants d’ordre 4. Nous examinons aussi quand les unités de K sont des sommes de deux carrés de K ou sont des carrés mod 4 de K.
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Acknowledgements
I would like to thank Richard Foote and Hershy Kisilevsky for many helpful conversations. I would also like to thank the anonymous referee, who passed along the observation that the squarefree part of \(\text {Norm}_{K/\mathbb {Q}} ( \varepsilon +1 )\) gives an integer m with \(m \varepsilon \) a square in K and suggested a complete answer for quadratic fields such as in Theorem 3 would be possible (rather than just examples of each type as in the original manuscript).
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Dummit, D.S. Classes of order 4 in the strict class group of number fields and remarks on unramified quadratic extensions of unit type. Ann. Math. Québec 43, 221–231 (2019). https://doi.org/10.1007/s40316-019-00112-7
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DOI: https://doi.org/10.1007/s40316-019-00112-7