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Cuspidal divisor class groups of non-split Cartan modular curves

Volume 187 / 2019

Pierfrancesco Carlucci Acta Arithmetica 187 (2019), 301-327 MSC: Primary 11G16; Secondary 11B68, 13C20. DOI: 10.4064/aa8516-6-2018 Published online: 8 February 2019


We find an explicit description of modular units in terms of Siegel functions for the modular curves $X^+_{\rm ns}(p^k) $ associated to the normalizer of a non-split Cartan subgroup of level $ p^k $ where $ p\not=2,3 $ is a prime. The cuspidal divisor class group $ \mathfrak{C}^+_{\rm ns}(p^k) $ on $X^+_{\rm ns}(p^k)$ is explicitly described as a module over the group ring $R = \mathbb{Z}[(\mathbb{Z}/p^k\mathbb{Z})^*/\{\pm 1\}] $. We give a formula for $ |\mathfrak{C}^+_{\rm ns}(p^k)| $ involving generalized Bernoulli numbers $ B_{2,\chi} $.


  • Pierfrancesco CarlucciDipartimento di Matematica
    Università degli Studi di Roma Tor Vergata
    Via della Ricerca Scientifica 1
    00133 Roma, Italy

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