# Tate curve

2010 Mathematics Subject Classification: *Primary:* 14H52 [MSN][ZBL]

A Tate curve is a uniformization of an elliptic curve having stable bad reduction with the help of a $q$-parametrization.

Let $K$ be a local field (e.g., $\mathbb{C}((t))$ or a finite extension of $\mathbb{Q}_p$). Let $E$ be an elliptic curve over $K$ such that it has stable reduction. Then it can have good reduction (i.e. with integral $j$-invariant) or bad reduction (i.e. with non-integral $j$-invariant). In the case of stable bad reduction one can construct an elliptic curve $E_q$ over $K$, which analytically is $K^*/q^{\mathbb{Z}}$ (where $q^{\mathbb{Z}}$ is the subgroup of $K^*$ generated by $q \in K^*$), such that $E$ and $E_q$ are isomorphic over a finite extension of $K$. One of the marvels of this theorem is the fact that the construction of the period $q$ starting from $E$, and the computation of the $j$-value of $E_q$, can be done without denominators (hence can be done in every characteristic): the $j$-value of the Tate curve with period $q$ is a power series in $q$ with coefficients in $\mathbb{Z}$: $$ j = q^{-1} + 744 + 196884 q + 21493760 q^2 + \cdots \ . $$

Such formulas can be found in [a4], Chapt. 15; [a8]. See also [a9], A.1.1, and [a10], Appendix C, Sect. 14. In [a2], VII, constructions over $\mathbb{Z}$ are given, with applications to compactifications of moduli schemes of elliptic curves. A generalization to higher-dimensional Abelian varieties over local fields with totally bad, stable reduction was given by H. Morikawa [a5], [a6] and by D. Mumford [a7]. This was generalized by G. Faltings and by C.-L. Chai to the case of stable reduction of Abelian varieties in [a3] and [a1], and it was used in the theory of compactifications of moduli schemes of Abelian varieties.

#### References

[a1] | C.-L. Chai, "Compactifications of Siegel moduli schemes" , Cambridge Univ. Press (1985) |

[a2] | P. Deligne, M. Rapoport, "Les schémas de modules de courbes elliptiques" P. Deligne (ed.) W. Kuyk (ed.) , Modular Functions II , Lect. notes in math. , 349 , Springer (1973) pp. 143–316 |

[a3] | G. Faltings, "Arithmetische Kompaktifizierung des Modulraums der abelschen Varietäten" F. Hirzebruch (ed.) J. Schwermer (ed.) S. Suter (ed.) , Arbeitstagung Bonn 1984 , Lect. notes in math. , 1111 , Springer (1985) pp. 321–383 |

[a4] | S. Lang, "Elliptic functions" , Addison-Wesley (1973) |

[a5] | H. Morikawa, "On theta functions and abelian varieties over valuation fields of rank one" Nagoya Math. J. , 20 (1962) pp. 1–27 |

[a6] | H. Morikawa, "On theta functions and abelian varieties over valuation fields of rank one" Nagoya Math. J. , 21 (1962) pp. 231–250 |

[a7] | D. Mumford, "An analytic construction of degenerating abelian varieties over complete rings" Compos. Math. , 24 (1972) pp. 129–174; 239–272 |

[a8] | P. Roquette, "Analytic theory of elliptic functions over local fields" , Vandenhoeck & Ruprecht (1970) |

[a9] | J.-P. Serre, "Abelian $\ell$-adic representations and elliptic curves" , Benjamin (1986) (Translated from French) |

[a10] | J.H. Silverman, "The arithmetic of elliptic curves" , Springer (1986) |

[a11] | C.-L. Chai, G. Faltings, "Semiabelian degeneration and compactification" , Forthcoming |

[a12] | S. Bosch, W. Lütkebohmert, M. Raynaud, "Néron models" , Springer (Forthcoming) |

[b1] | Alain Robert, "Elliptic curves. Notes from postgraduate lectures given in Lausanne 1971/72", Lecture Notes in Mathematics 326 Springer (1973) Zbl 0256.14013 |

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Tate curve.

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