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A note on integral bases of unramified cyclic extensions of prime degree. (English) Zbl 1018.11055

Let \(L/K\) be a finite extension of number fields, unramified at all finite primes, with rings of integers \(O_L, O_K\) and Galois group \(\Gamma\). If \(O_L = O_K[\alpha]\) for some \(\alpha \in O_L\), \(L/K\) is said to have a relative power integral basis (PIB). If \(O_L = O_K[\Gamma](\beta)\) for some \(\beta \in O_L\), \(L/K\) is said to have a relative normal integral basis (NIB). When \(\Gamma\) is cyclic of prime order \(p\) and \(K\) contains a primitive \(p\)th root of unity \(\zeta_p\), the reviewer [Proc. Lond. Math. Soc. (3) 35, 407-422 (1977; Zbl 0374.13002)] showed that if \(L/K\) has a NIB, it has a PIB. The author obtains infinitely many cubic extensions \(L/K\) with \(\zeta_3 \in K\) with a PIB but no NIB, and also infinitely many quadratic extensions \(L/K\) with PIB but no NIB where \(K\) is a real quadratic field. The examples build on earlier results of the author [J. Algebra 235, 104-112 (2001; Zbl 0972.11101)].

MSC:

11R33 Integral representations related to algebraic numbers; Galois module structure of rings of integers
11R11 Quadratic extensions
Full Text: DOI

References:

[1] Childs, L., The group of unramified Kummer extensions of prime degree, Proc. London Math. Soc., 35, 407-422 (1977) · Zbl 0374.13002 · doi:10.1112/plms/s3-35.3.407
[2] H. Ichimura, On power integral bases of unramified cyclic extensions of prime degree. To appear inJournal of Algebra. · Zbl 0972.11101
[3] -, On a power integral bases problem over cyclotomic ℤ_p-extensions. To appear inJournal of Algebra. · Zbl 0990.11060
[4] S. Lang,Algebraic Number Theory. Springer, 1986. · Zbl 0601.12001
[5] L. Washington,Introduction to Cyclotomic Fields (2-nd edition). Springer, 1996. · Zbl 0484.12001
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