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In mathematics, a p-adic zeta function, or more generally a p-adic L-function, is a function analogous to the Riemann zeta function, or more general L-functions, but whose domain and target are p-adic (where p is a prime number). For example, the domain could be the p-adic integers Zp, a profinite p-group, or a p-adic family of Galois representations, and the image could be the p-adic numbers Qp or its algebraic closure.

Dirichlet L-functions

The Dirichlet L-function is given by the analytic continuation of

$$L(s,\chi) = \sum_n\frac{\chi(n)}{n^s} = \prod_{p \text{ prime}} \frac{1}{1-\chi(p)p^{-s}}$$

The Dirichlet L-function at negative integers is given by

$$L(1-n, \chi) = -\frac{B_{n,\chi}}{n}$$

where Bn,χ is a generalized Bernoulli number defined by

$$\displaystyle \sum_{n=0}^\infty B_{n,\chi}\frac{t^n}{n!} = \sum_{a=1}^f\frac{\chi(a)te^{at}}{e^{ft}-1}$$

for χ a Dirichlet character with conductor f.
Definition using interpolation

The Kubota–Leopoldt p-adic L-function Lp(s, χ) interpolates the Dirichlet L-function with the Euler factor at p removed. More precisely, Lp(s,χ) is the unique continuous function of the p-adic number s such that

$$\displaystyle L_p(1-n, \chi) = (1-\chi(p)p^{n-1})L(1-n, \chi)$$

for positive integers n divisible by p−1. The right hand side is just the usual Dirichlet L-function, except that the Euler factor at p is removed, otherwise it would not be p-adically continuous. The continuity of the right hand side is closely related to the Kummer congruences.

When n is not divisible by p−1 this does not usually hold; instead

$$\displaystyle L_p(1-n, \chi) = (1-\chi\omega^{-n}(p)p^{n-1})L(1-n, \chi\omega^{-n})$$

for positive integers n. Here χ is twisted by a power of the Teichmuller character ω.

p-adic L-functions can also be thought of as p-adic measures (or p-adic distributions) on p-profinite Galois groups. The translation between this point of view and the original point of view of Kubota–Leopoldt (as Qp-valued functions on Zp) is via the Mazur–Mellin transform (and class field theory).
Totally real fields

Deligne & Ribet (1980), building upon previous work of Serre (1973), constructed analytic p-adic L-functions for totally real fields. Independently, Barsky (1978) and Cassou-Noguès (1979) did the same, but their approaches followed Takuro Shintani's approach to the study of the L-values.
References

Barsky, Daniel (1978), "Fonctions zeta p-adiques d'une classe de rayon des corps de nombres totalement réels", in Amice, Y.; Barskey, D.; Robba, P., Groupe d'Etude d'Analyse Ultramétrique (5e année: 1977/78), 16, Paris: Secrétariat Math., ISBN 978-2-85926-266-2, MR525346
Cassou-Noguès, Pierrette (1979), "Valeurs aux entiers négatifs des fonctions zêta et fonctions zêta p-adiques", Inventiones Mathematicae 51 (1): 29–59, doi:10.1007/BF01389911, ISSN 0020-9910, MR524276
Coates, John (1989), "On p-adic L-functions", Astérisque (177): 33–59, ISSN 0303-1179, MR1040567
Colmez, Pierre (2004), Fontaine's rings and p-adic L-functions
Deligne, Pierre; Ribet, Kenneth A. (1980), "Values of abelian L-functions at negative integers over totally real fields", Inventiones Mathematicae 59 (3): 227–286, doi:10.1007/BF01453237, ISSN 0020-9910, MR579702
Iwasawa, Kenkichi (1969), "On p-adic L-functions", Annals of Mathematics. Second Series (Annals of Mathematics) 89 (1): 198–205, doi:10.2307/1970817, ISSN 0003-486X, JSTOR 1970817, MR0269627
Iwasawa, Kenkichi (1972), Lectures on p-adic L-functions, Princeton University Press, ISBN 978-0-691-08112-0, MR0360526
Katz, Nicholas M. (1975), "p-adic L-functions via moduli of elliptic curves", Algebraic geometry, Proc. Sympos. Pure Math.,, 29, Providence, R.I.: American Mathematical Society, pp. 479–506, MR0432649
Koblitz, Neal (1984), p-adic Numbers, p-adic Analysis, and Zeta-Functions, Graduate Texts in Mathematics, vol. 58, Berlin, New York: Springer-Verlag, ISBN 978-0-387-96017-3, MR754003
Kubota, Tomio; Leopoldt, Heinrich-Wolfgang (1964), "Eine p-adische Theorie der Zetawerte. I. Einführung der p-adischen Dirichletschen L-Funktionen", Journal für die reine und angewandte Mathematik 214/215: 328–339, ISSN 0075-4102, MR0163900
Serre, Jean-Pierre (1973), "Formes modulaires et fonctions zêta p-adiques", in Kuyk, Willem; Serre, Jean-Pierre, Modular functions of one variable, III (Proc. Internat. Summer School, Univ. Antwerp, 1972), Lecture Notes in Math, 350, Berlin, New York: Springer-Verlag, pp. 191–268, doi:10.1007/978-3-540-37802-0_4, ISBN 978-3-540-06483-1, MR0404145