 Dedekind zeta function

In mathematics, the Dedekind zeta function of an algebraic number field K, generally denoted ζ_{K}(s), is a generalization of the Riemann zeta function—which is obtained by specializing to the case where K is the rational numbers Q. In particular, it can be defined as a Dirichlet series, it has an Euler product expansion, it satisfies a functional equation, it has an analytic continuation to a meromorphic function on the complex plane C with only a simple pole at s = 1, and its values encode arithmetic data of K. The extended Riemann hypothesis states that if ζ_{K}(s) = 0 and 0 < Re(s) < 1, then Re(s) = 1/2.
The Dedekind zeta function is named for Richard Dedekind who introduced them in his supplement to P.G.L. Dirichlet's Vorlesungen über Zahlentheorie.^{[1]}
Contents
Definition and basic properties
Let K be an algebraic number field K. Its Dedekind zeta function is first defined for complex numbers s with real part Re(s) > 1 by the Dirichlet series
where I ranges through the nonzero ideals of the ring of integers O_{K} of K and N_{K/Q}(I) denotes the absolute norm of I (which is equal to both the index [O_{K} : I] of I in O_{K} or equivalently the cardinality of quotient ring O_{K} / I). This sum converges absolutely for all complex numbers s with real part Re(s) > 1. In the case K = Q, this definition reduces to that of the Riemann zeta function.
Euler product
The Dedekind zeta function of K has an Euler product which is a product over all the prime ideals P of O_{K}
This is the expression in analytic terms of the uniqueness of prime factorization of the ideals I in O_{K}. The fact that, for Re(s) > 1, ζ_{K}(s) is given by a product of nonzero numbers implies that it is nonzero in this region.
Analytic continuation and functional equation
Erich Hecke first proved that ζ_{K}(s) has an analytic continuation to the complex plane as a meromorphic function, having a simple pole only at s = 1. The residue at that pole is given by the analytic class number formula and is made up of important arithmetic data involving invariants of the unit group and class group of K.
The Dedekind zeta function satisfies a functional equation relating its values at s and 1 − s. Specifically, let Δ_{K} denote discriminant of K, let r_{1} (resp. r_{2}) denote the number of real places (resp. complex places) of K, and let
and
where Γ(s) is the Gamma function. Then, the function
satisfies the functional equation
 Λ_{K}(s) = Λ_{K}(1 − s).
Special values
Analogously to the Riemann zeta function, the values of the Dedekind zeta function at integers encode (at least conjecturally) important arithmetic data of the field K. For example, the analytic class number formula relates the residue at s = 1 to the class number h(K) of K, the regulator R(K) of K, the number w(K) of roots of unity in K, the absolute discriminant of K, and the number of real and complex places of K. Another example is at s = 0 where it has a zero whose order r is equal to the rank of the unit group of O_{K} and the leading term is given by
Combining the functional equation and the fact that Γ(s) is zero at all integers less than or equal to zero yields that ζ_{K}(s) vanishes at all negative even integers. It even vanishes at all negative odd integers unless K is totally real (i.e. r_{2} = 0; e.g. Q or a real quadratic field). In the totally real case, Carl Ludwig Siegel showed that ζ_{K}(s) is a nonzero rational number at negative odd integers. Stephen Lichtenbaum conjectured specific values for these rational numbers in terms of the algebraic Ktheory of K.
Relations to other Lfunctions
For the case in which K is an abelian extension of Q, its Dedekind zeta function can be written as a product of Dirichlet Lfunctions. For example, when K is a quadratic field this shows that the ratio
is the Lfunction L(s, χ), where χ is a Jacobi symbol as Dirichlet character. That the zeta function of a quadratic field is a product of the Riemann zeta function and a certain Dirichlet Lfunction is an analytic formulation of the quadratic reciprocity law of Gauss.
In general, if K is a Galois extension of Q with Galois group G, its Dedekind zeta function is the Artin Lfunction of the regular representation of G and hence has a factorization in terms of Artin Lfunctions of irreducible Artin representations of G.
Additionally, ζ_{K}(s) is the Hasse–Weil zeta function of Spec O_{K}^{[2]} and the motivic Lfunction of the motive coming from the cohomology of Spec K.^{[3]}
Arithmetically equivalent fields
Two fields are called arithmetically equivalent if they have the same Dedekind zeta function. Wieb Bosma and Bart de Smit (2002) used Gassmann triples to give some examples of pairs of nonisomorphic fields that are arithmetically equivalent. In particular some of these pairs have different class numbers, so the Dedekind zeta function of a number field does not determine its class number.
Notes
 ^ Narkiewicz 2004, §7.4.1
 ^ Deninger 1994, §1
 ^ Flach 2004, §1.1
References
 Bosma, Wieb; de Smit, Bart (2002), "On arithmetically equivalent number fields of small degree", in Kohel, David R.; Fieker, Claus, Algorithmic number theory (Sydney, 2002), Lecture Notes in Comput. Sci., 2369, Berlin, New York: SpringerVerlag, pp. 67–79, doi:10.1007/3540454551_6, ISBN 9783540438632, MR2041074, http://dx.doi.org/10.1007/3540454551_6
 Section 10.5.1 of Cohen, Henri (2007), Number theory, Volume II: Analytic and modern tools, Graduate Texts in Mathematics, 240, New York: Springer, doi:10.1007/9780387498942, ISBN 9780387498935, MR2312338
 Deninger, Christopher (1994), "Lfunctions of mixed motives", in Jannsen, Uwe; Kleiman, Steven; Serre, JeanPierre, Motives, Part 1, Proceedings of Symposia in Pure Mathematics, 55.1, American Mathematical Society, pp. 517–525, ISBN 9780821816356, http://wwwmath.unimuenster.de/u/deninger/about/publikat/cd22.ps
 Flach, Mathias, "The equivariant Tamagawa number conjecture: a survey", in Burns, David; Popescu, Christian; Sands, Jonathan et al., Stark's conjectures: recent work and new directions, Contemporary Mathematics, 358, American Mathematical Society, pp. 79–125, ISBN 9780821834800, http://www.math.caltech.edu/papers/baltimorefinal.pdf
 Chapter 7 of Narkiewicz, Władysław (2004), Elementary and analytic theory of algebraic numbers, Springer Monographs in Mathematics (3 ed.), Berlin: SpringerVerlag, ISBN 9783540219026, MR2078267
Lfunctions in number theory Analytic examples Algebraic examples Theorems Analytic class number formula • Weil conjecturesAnalytic conjectures Riemann hypothesis • Generalized Riemann hypothesis • Lindelöf hypothesis • Ramanujan–Petersson conjecture • Artin conjectureAlgebraic conjectures Birch and SwinnertonDyer conjecture • Deligne's conjecture • Beilinson conjectures • Bloch–Kato conjecture • Langlands conjecturepadic Lfunctions Main conjecture of Iwasawa theory • Selmer group • Euler systemCategories: Zeta and Lfunctions
 Algebraic number theory
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