Discriminant of an algebraic number field

Discriminant of an algebraic number field
A fundamental domain of the ring of integers of the field K obtained from Q by adjoining a root of x3 − x2 − 2x + 1. This fundamental domain sits inside K ⊗QR. The discriminant of K is 49 = 72. Accordingly, the volume of the fundamental domain is 7 and K is only ramified at 7.

In mathematics, the discriminant of an algebraic number field is a numerical invariant that, loosely speaking, measures the size of the (ring of integers of the) algebraic number field. More specifically, it is related to the volume of the fundamental domain of the ring of integers, and it regulates which primes are ramified.

The discriminant is one of the most basic invariants of a number field, and occurs in several important analytic formulas such as the functional equation of the Dedekind zeta function of K, and the analytic class number formula for K. An old theorem of Hermite states that there are only finitely many number fields of bounded discriminant, however determining this quantity is still an open problem, and the subject of current research.[1]

The discriminant of K can be referred to as the absolute discriminant of K to distinguish it from the relative discriminant of an extension K/L of number fields. The latter is an ideal in the ring of integers of L, and like the absolute discriminant it indicates which primes are ramified in K/L. It is a generalization of the absolute discriminant allowing for L to be bigger than Q; in fact, when L = Q, the relative discriminant of K/Q is the principal ideal of Z generated by the absolute discriminant of K.

Contents

Definition

Let K be an algebraic number field, and let OK be its ring of integers. Let b1, ..., bn be an integral basis of OK (i.e. a basis as a Z-module), and let {σ1, ..., σn} be the set of embeddings of K into the complex numbers (i.e. ring homomorphisms K → C). The discriminant of K is the square of the determinant of the n by n matrix B whose (i,j)-entry is σi(bj). Symbolically,

\Delta_K=\operatorname{det}\left(\begin{array}{cccc} \sigma_1(b_1) & \sigma_1(b_2) &\cdots & \sigma_1(b_n) \\ \sigma_2(b_1) & \ddots & & \vdots \\ \vdots & & \ddots & \vdots \\ \sigma_n(b_1) & \cdots & \cdots & \sigma_n(b_n) \end{array}\right)^2.


Equivalently, the trace from K to Q can be used. Specifically, define the trace form to be the matrix whose (i,j)-entry is TrK/Q(bibj). This matrix equals BTB, so the discriminant of K is the determinant of this matrix.

Examples

\Delta_K=\left\{\begin{array}{ll} d &\mbox{if }d\equiv 1\mbox{ mod }4 \\ 4d &\mbox{if }d\equiv 2,3\mbox{ mod }4. \\\end{array}\right.
An integer that occurs as the discriminant of a quadratic number field is called a fundamental discriminant.[2]
\Delta_{K_n} = (-1)^{\varphi(n)/2} \frac{n^{\varphi(n)}}{\displaystyle\prod_{p|n} p^{\varphi(n)/(p-1)}}
where φ(n) is Euler's totient function, and the product in the denominator is over primes p dividing n.
  • Power bases: In the case where the ring of integers has a power integral basis, that is, can be written as OK = Z[α], the discriminant of K is equal to the discriminant of the minimal polynomial of α. To see this, one can chose the integral basis of OK to be b1 = 1, b2 = α, b3 = α2, ..., bn = αn−1. Then, the matrix in the definition is the Vandermonde matrix associated to αi = σi(α), whose determinant squared is
\prod_{1\leq i<j\leq n}(\alpha_i-\alpha_j)^2
which is exactly the definition of the discriminant of the minimal polynomial.
  • Let K = Q(α) be the number field obtained by adjoining a root α of the polynomial x3 − x2 − 2x − 8. This is Richard Dedekind's original example of a number field whose ring of integers does not possess a power basis. An integral basis is given by {1, α, α(α + 1)/2} and the discriminant of K is −503.[4][5]
  • Repeated discriminants: the discriminant of a quadratic field uniquely identifies it, but this is not true, in general, for higher-degree number fields. For example, there are two non-isomorphic cubic fields of discriminant 3969. They are obtained by adjoining a root of the polynomial x3 − 21x + 28 or x3 − 21x − 35, respectively.[6]

Basic results

  • The sign of the discriminant is (−1)r2 where r2 is the number of complex places of K.[7]
  • A prime p ramifies in K if, and only if, p divides ΔK .[8]
  • Stickelberger's Theorem:[9]
\Delta_K\equiv 0\mbox{ or }1 \mbox{ mod 4}
  • Minkowski bound:[10] Let n denote the degree of the extension K/Q and r2 the number of complex places of K, then
|\Delta_K|^{1/2}\geq \frac{n^n}{n!}\left(\frac{\pi}{4}\right)^{r_2} \geq \frac{n^n}{n!}\left(\frac{\pi}{4}\right)^{n/2}.
  • Minkowski's Theorem:[11] If K is not Q, then |ΔK| > 1 (this follows directly from the Minkowski bound).
  • Hermite's Theorem:[12] Let N be a positive integer. There are only finitely many algebraic number fields K with ΔK < N.

History

Richard Dedekind showed that every number field possesses an integral basis, allowing him to define the discriminant of an arbitrary number field.[13]

The definition of the discriminant of a general algebraic number field, K, was given by Dedekind in 1871.[13] At this point, he already knew the relationship between the discriminant and ramification.[14]

Hermite's theorem predates the general definition of the discriminant with Charles Hermite publishing a proof of it in 1857.[15] In 1877, Alexander von Brill determined the sign of the discriminant.[16] Leopold Kronecker first stated Minkowski's theorem in 1882,[17] though the first proof was given by Hermann Minkowski in 1891.[18] In the same year, Minkowski published his bound on the discriminant.[19] Near the end of the nineteenth century, Ludwig Stickelberger obtained his theorem on the residue of the discriminant modulo four.[20][21]

Relative discriminant

The discriminant defined above is sometimes referred to as the absolute discriminant of K to distinguish it from the relative discriminant ΔK/L of an extension of number fields K/L, which is an ideal in OL. The relative discriminant is defined in a fashion similar to the absolute discriminant, but must take into account that ideals in OL may not be principal and that there may not be an integral basis of K/L. Let {σ1, ..., σn} be the set of embeddings of K into C which are the identity on L. If b1, ..., bn is any basis of K over L, let d(b1, ..., bn) be the square of the determinant of the n by n matrix whose (i,j)-entry is σi(bj). Then, the relative discriminant of K/L is the ideal generated by the d(b1, ..., bn) as {b1, ..., bn} varies over all bases of K/L with the property that bi ∈ OK for all i. Alternatively, the relative discriminant of K/L is the norm of the different of K/L.[22] When L = Q, the relative discriminant ΔK/Q is the principal ideal of Z generated by the absolute discriminant ΔK . In a tower of fields K/L/F the relative discriminants are related by

\Delta_{K/F} = \mathcal{N}_{L/F}\left({\Delta_{K/L}}\right) \Delta_{L/F}^{[K:L]}

where \mathcal{N} denotes relative norm.[23]

Ramification

The relative discriminant regulates the ramification data of the field extension K/L. A prime ideal p of L ramifies in K if, and only if, it divides the relative discriminant ΔK/L. An extension is unramified if, and only if, the discriminant is the unit ideal.[22] The Minkowski bound above shows that there are no non-trivial unramified extensions of Q. Fields larger than Q may have unramified extensions, for example, for any field with class number greater than one, its Hilbert class field is a non-trivial unramified extension.

Root discriminant

The root discriminant of a number field, K, of degree n, often denoted rdK, is defined as the n-th root of the absolute value of the (absolute) discriminant of K.[24] The relation between relative discriminants in a tower of fields shows that the root discriminant does not change in an unramified extension.

Relation to other quantities

  • When embedded into K\otimes_\mathbf{Q}\mathbf{R}, the volume of the fundamental domain of OK is \sqrt{|\Delta_K|} (sometimes a different measure is used and the volume obtained is 2^{-r_2}\sqrt{|\Delta_K|}, where r2 is the number of complex places of K).
  • Due to its appearance in this volume, the discriminant also appears in the functional equation of the Dedekind zeta function of K, and hence in the analytic class number formula, and the Brauer-Siegel theorem.
  • The relative discriminant of K/L is the Artin conductor of the regular representation of the Galois group of K/L. This provides a relation to the Artin conductors of the characters of the Galois group of K/L, called the conductor-discriminant formula.[25]

Notes

  1. ^ Cohen, Diaz y Diaz & Olivier 2002
  2. ^ Definition 5.1.2 of Cohen 1993
  3. ^ Proposition 2.7 of Washington 1997
  4. ^ Dedekind 1878, pp. 30–31
  5. ^ Narkiewicz 2004, p. 64
  6. ^ Cohen 1993, Theorem 6.4.6
  7. ^ Lemma 2.2 of Washington 1997
  8. ^ Corollary III.2.12 of Neukirch 1999
  9. ^ Exercise I.2.7 of Neukirch 1999
  10. ^ Proposition III.2.14 of Neukirch 1999
  11. ^ Theorem III.2.17 of Neukirch 1999
  12. ^ Theorem III.2.16 of Neukirch 1999
  13. ^ a b Dedekind's supplement X of the second edition of Dirichlet's Vorlesungen über Zahlentheorie (Dedekind 1871)
  14. ^ Bourbaki 1994
  15. ^ Hermite 1857.
  16. ^ Brill 1877.
  17. ^ Kronecker 1882.
  18. ^ Minkowski 1891a.
  19. ^ Minkowski 1891b.
  20. ^ Stickelberger 1897.
  21. ^ All facts in this paragraph can be found in Narkiewicz 2004, pp. 59, 81
  22. ^ a b Neukirch 1999, §III.2
  23. ^ Corollary III.2.10 of Neukirch 1999 or Proposition III.2.15 of Fröhlich & Taylor 1993
  24. ^ Voight 2008
  25. ^ Section 4.4 of Serre 1967

References

Primary sources

Secondary sources

  • Bourbaki, Nicolas (1994), Elements of the history of mathematics, Berlin: Springer-Verlag, ISBN 978-3-540-64767-6, MR1290116 . Translated from the original French by John Meldrum
  • Cohen, Henri (1993), A Course in Computational Algebraic Number Theory, Graduate Texts in Mathematics, 138, Berlin, New York: Springer-Verlag, ISBN 978-3-540-55640-4, MR1228206 
  • Cohen, Henri; Diaz y Diaz, Francisco; Olivier, Michel (2002), "A Survey of Discriminant Counting", in Fieker, Claus; Kohel, David R., Algorithmic Number Theory, Proceedings, 5th International Syposium, ANTS-V, University of Sydney, July 2002, Lecture Notes in Computer Science, 2369, Berlin: Springer-Verlag, pp. 80–94, doi:10.1007/3-540-45455-1_7, ISBN 978-3-540-43863-2, ISSN 0302-9743, MR2041075, http://www.springerlink.com/content/2bl9qep1qb4k32vy/, retrieved 2009-08-19 
  • Fröhlich, Albrecht; Taylor, Martin (1993), Algebraic number theory, Cambridge Studies in Advanced Mathematics, 27, Cambridge University Press, ISBN 978-0-521-43834-6, MR1215934 
  • Narkiewicz, Władysław (2004), Elementary and analytic theory of algebraic numbers, Springer Monographs in Mathematics (3 ed.), Berlin: Springer-Verlag, ISBN 978-3-540-21902-6, MR2078267 
  • Neukirch, Jürgen (1999), Algebraic Number Theory, Grundlehren der mathematischen Wissenschaften, 322, Berlin: Springer-Verlag, ISBN 978-3-540-65399-8, MR1697859 
  • Serre, Jean-Pierre (1967), "Local class field theory", in Cassels, J. W. S.; Fröhlich, Albrecht, Algebraic Number Theory, Proceedings of an instructional conference at the University of Sussex, Brighton, 1965, London: Academic Press, ISBN 0-121-63251-2, MR0220701 
  • Voight, John (2008), "Enumeration of totally real number fields of bounded root discriminant", in van der Poorten, Alfred J.; Stein, Andreas, Algorithmic number theory. Proceedings, 8th International Symposium, ANTS-VIII, Banff, Canada, May 2008, Lecture Notes in Computer Science, 5011, Berlin: Springer-Verlag, pp. 268–281, arXiv:0802.0194, doi:10.1007/978-3-540-79456-1_18, ISBN 978-3-540-79455-4, MR2467853 
  • Washington, Lawrence (1997), Introduction to Cyclotomic Fields, Graduate Texts in Mathematics, 83 (2 ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-94762-4, MR1421575 

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