Abstract
Let K be a number field and G a finitely generated torsion-free subgroup of \(K^\times \). Given a prime \(\mathfrak {p}\) of K we denote by \({{\,\textrm{ind}\,}}_\mathfrak {p}(G)\) the index of the subgroup \((G\bmod \mathfrak {p})\) of the multiplicative group of the residue field at \(\mathfrak {p}\). Under the Generalized Riemann Hypothesis we determine the natural density of primes of K for which this index is in a prescribed set S and has prescribed Frobenius in a finite Galois extension F of K. We study in detail the natural density in case S is an arithmetic progression, in particular its positivity.
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Acknowledgements
This project was started when the first author gave a talk in the Luxembourg Number Theory Day 2019. He thanks the other authors for the invitation and for several subsequent invitations. The second and third author thank the Max Planck Institut für Mathematik and the first author for organizing a short visit in October 2022. Thanks are also due to Alessandro Languasco for verifying Table 2 using Theorem 16. We thank Valentin Blomer for pointing out reference [1].
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Moree, P., Perucca, A. & Sgobba, P. The distribution of the multiplicative index of algebraic numbers over residue classes. Abh. Math. Semin. Univ. Hambg. (2024). https://doi.org/10.1007/s12188-024-00276-2
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DOI: https://doi.org/10.1007/s12188-024-00276-2
Keywords
- Reductions of algebraic numbers
- Multiplicative index and order
- Primes in arithmetic progression
- Natural density