Modulus (algebraic number theory)

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In mathematics, in the field of algebraic number theory, a modulus (plural moduli) (or cycle,[1] or extended ideal[2]) is a formal product of places of a global field (i.e. an algebraic number field or a global function field). It is used to encode ramification data for abelian extensions of a global field.


Let K be a global field with ring of integers R. A modulus is a formal product[3][4]

where p runs over all places of K, finite or infinite, the exponents ν(p) are zero except for finitely many p. If K is a number field, ν(p) = 0 or 1 for real places and ν(p) = 0 for complex places. If K is a function field, ν(p) = 0 for all infinite places.

In the function field case, a modulus is the same thing as an effective divisor,[5] and in the number field case, a modulus can be considered as special form of Arakelov divisor.[6]

The notion of congruence can be extended to the setting of moduli. If a and b are elements of K×, the definition of a ≡b (mod pν) depends on what type of prime p is:[7][8]

  • if it is finite, then
where ordp is the normalized valuation associated to p;
  • if it is a real place (of a number field) and ν = 1, then
under the real embedding associated to p.
  • if it is any other infinite place, there is no condition.

Then, given a modulus m, a ≡b (mod m) if a ≡b (mod pν(p)) for all p such that ν(p) > 0.

Ray class group[edit]

The ray modulo m is[9][10][11]

A modulus m can be split into two parts, mf and m, the product over the finite and infinite places, respectively. Let Im to be one of the following:

In both case, there is a group homomorphism i : Km,1Im obtained by sending a to the principal ideal (resp. divisor) (a).

The ray class group modulo m is the quotient Cm = Im / i(Km,1).[14][15] A coset of i(Km,1) is called a ray class modulo m.

Erich Hecke's original definition of Hecke characters may be interpreted in terms of characters of the ray class group with respect to some modulus m.[16]


When K is a number field, the following properties hold.[17]

  • When m = 1, the ray class group is just the ideal class group.
  • The ray class group is finite. Its order is the ray class number.
  • The ray class number is divisible by the class number of K.


  1. ^ Lang 1994, §VI.1
  2. ^ Cohn 1985, definition 7.2.1
  3. ^ Janusz 1996, §IV.1
  4. ^ Serre 1988, §III.1
  5. ^ Serre 1988, §III.1
  6. ^ Neukirch 1999, §III.1
  7. ^ Janusz 1996, §IV.1
  8. ^ Serre 1988, §III.1
  9. ^ Milne 2008, §V.1
  10. ^ Janusz 1996, §IV.1
  11. ^ Serre 1988, §VI.6
  12. ^ Janusz 1996, §IV.1
  13. ^ Serre 1988, §V.1
  14. ^ Janusz 1996, §IV.1
  15. ^ Serre 1988, §VI.6
  16. ^ Neukirch 1999, §VII.6
  17. ^ Janusz, 1996 & §4.1


  • Cohn, Harvey (1985), Introduction to the construction of class fields, Cambridge studies in advanced mathematics, 6, Cambridge University Press, ISBN 978-0-521-24762-7
  • Janusz, Gerald J. (1996), Algebraic number fields, Graduate Studies in Mathematics, 7, American Mathematical Society, ISBN 978-0-8218-0429-2
  • Lang, Serge (1994), Algebraic number theory, Graduate Texts in Mathematics, 110 (2 ed.), New York: Springer-Verlag, ISBN 978-0-387-94225-4, MR 1282723
  • Milne, James (2008), Class field theory (v4.0 ed.), retrieved 2010-02-22
  • Neukirch, Jürgen (1999). Algebraic Number Theory. Grundlehren der mathematischen Wissenschaften. 322. Berlin: Springer-Verlag. ISBN 978-3-540-65399-8. MR 1697859. Zbl 0956.11021.
  • Serre, Jean-Pierre (1988), Algebraic groups and class fields, Graduate Texts in Mathematics, 117, New York: Springer-Verlag, ISBN 978-0-387-96648-9