# Dirichlet character

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In mathematics, specifically number theory, **Dirichlet characters** are certain arithmetic functions which arise from completely multiplicative characters on the units of . Dirichlet characters are used to define Dirichlet *L*-functions, which are meromorphic functions with a variety of interesting analytic properties.

If is a Dirichlet character, one defines its Dirichlet *L*-series by

where *s* is a complex number with real part > 1. By analytic continuation, this function can be extended to a meromorphic function on the whole complex plane. Dirichlet *L*-functions are generalizations of the Riemann zeta-function and appear prominently in the generalized Riemann hypothesis.

Dirichlet characters are named in honour of Peter Gustav Lejeune Dirichlet. They were later generalized by Erich Hecke to Hecke characters (also known as Grössencharacter).

## Axiomatic definitionEdit

We say that a function from the integers to the complex numbers is a Dirichlet character
if it has the following properties:^{[1]}

- There exists a positive integer
*k*such that χ(*n*) = χ(*n*+*k*) for all integers*n*. - If gcd(
*n*,*k*) > 1 then χ(*n*) = 0; if gcd(*n*,*k*) = 1 then χ(*n*) ≠ 0. - χ(
*mn*) = χ(*m*) χ(*n*) for all integers*m*and*n*.

From this definition, several other properties can be deduced.
By property 3, χ(1) = χ(1 × 1) = χ(1) χ(1). Since gcd(1, *k*) = 1, property 2 says χ(1) ≠ 0, so

- χ(1) = 1.

Properties 3 and 4 show that every Dirichlet character χ is completely multiplicative.

Property 1 says that a character is periodic with period *k*; we say that is a character to the **modulus** *k*. This is equivalent to saying that

- If
*a*≡*b*(mod*k*) then χ(*a*) = χ(*b*).

If gcd(*a*, *k*) = 1, Euler's theorem says that *a*^{φ(k)} ≡ 1 (mod *k*) (where φ(*k*) is the totient function). Therefore, by properties 5 and 4, χ(*a*^{φ(k)}) = χ(1) = 1, and by 3, χ(*a*^{φ(k)}) =χ(*a*)^{φ(k)}. So

- For all
*a*relatively prime to*k*, χ(*a*) is a φ(*k*)-th complex root of unity, i.e. for some integer 0 ≤*r*< φ(*k*).

The unique character of period 1 is called the **trivial character**. Note that any character vanishes at 0 except the trivial one, which is 1 on all integers.

A character is called **principal** if it assumes the value 1 for arguments coprime to its modulus and otherwise is 0.^{[2]} A character is called **real** if it assumes real values only. A character which is not real is called **complex**.^{[3]}

The **sign** of the character depends on its value at −1. Specifically, is said to be **odd** if and **even** if .

## Construction via residue classesEdit

Dirichlet characters may be viewed in terms of the character group of the group of units of the ring **Z**/*k***Z**, as *extended residue class characters*.^{[4]}

### Residue classesEdit

Given an integer *k*, one defines the **residue class** of an integer *n* as the set of all integers congruent to *n* modulo *k*:
That is, the residue class is the coset of *n* in the quotient ring **Z**/*k***Z**.

The set of units modulo *k* forms an abelian group of order , where group multiplication is given by
and
again denotes Euler's phi function.
The identity in this group is the residue class and the inverse of is the residue class where
, i.e., . For example, for *k*=6, the set of units is because 0, 2, 3, and 4 are not coprime to 6.

The character group of (**Z**/*k*)^{*} consists of the *residue class characters*. A residue class character θ on (**Z**/*k*)^{*} is **primitive** if there is no proper divisor *d* of *k* such that θ factors as a map (**Z**/*k*)^{*} → (**Z**/*d*)^{*} → **C**^{*}, where the first arrow is the natural "modding *d* " map.^{[5]}

### Dirichlet charactersEdit

The definition of a Dirichlet character modulo *k* ensures that it restricts to a character of the unit group modulo *k*:^{[6]} a group homomorphism from (**Z**/*k***Z**)^{*} to the non-zero complex numbers

- ,

with values that are necessarily roots of unity since the units modulo *k* form a finite group. In the opposite direction, given a group homomorphism on the unit group modulo *k*, we can lift to a completely multiplicative function on integers relatively prime to *k* and then extend this function to all integers by defining it to be 0 on integers having a non-trivial factor in common with *k*. The resulting function will then be a Dirichlet character.^{[7]}

The **principal character** modulo *k* has the properties^{[7]}

- if gcd(
*n*,*k*) = 1 and - if gcd(
*n*,*k*) > 1.

The associated character of the multiplicative group (**Z**/*k***Z**)^{*} is the *principal* character which always takes the value 1.^{[8]}

When *k* is 1, the principal character modulo *k* is equal to 1 at all integers. For *k* greater than 1, the principal character modulo *k* vanishes at integers having a non-trivial common factor with *k* and is 1 at other integers.

There are φ(*n*) Dirichlet characters modulo *n*.^{[7]}

## Equivalent definitionsEdit

There are several ways of defining Dirichlet characters, based on other properties that these functions satisfy.

### Sárközy's Condition^{[9]}Edit

A Dirichlet character is a completely multiplicative function that satisfies a linear recurrence relation: that is, if

for all positive integer , where are not all zero and are distinct then is a Dirichlet character.

### Chudakov's ConditionEdit

A Dirichlet character is a completely multiplicative function satisfying the following three properties: a) takes only finitely many values; b) vanishes at only finitely many primes; c) there is an for which the remainder

is uniformly bounded, as . This equivalent definition of Dirichlet characters was conjectured by Chudakov^{[10]} in 1956, and proved in 2017 by Klurman and Mangerel.^{[11]}

## A few character tablesEdit

The tables below help illustrate the nature of a Dirichlet character. They present all of the characters from modulus 1 to modulus 12. The characters χ_{0} are the principal characters.

### Modulus 1Edit

There is character modulo 1:

χ \ *n***0**1

Note that χ is wholly determined by χ(0) since 0 generates the group of units modulo 1.

This is the trivial character.

The Dirichlet *L*-series for is the Riemann zeta function

- .

### Modulus 2Edit

There is character modulo 2:

χ \ *n***0****1**0 1

Note that χ is wholly determined by χ(1) since 1 generates the group of units modulo 2.

The Dirichlet *L*-series for is
the Dirichlet lambda function (closely related to the Dirichlet eta function)

### Modulus 3Edit

There are characters modulo 3:

χ \ *n***0****1****2**0 1 1 0 1 −1

Note that χ is wholly determined by χ(2) since 2 generates the group of units modulo 3.

### Modulus 4Edit

There are characters modulo 4:

χ \ *n***0****1****2****3**0 1 0 1 0 1 0 −1

Note that χ is wholly determined by χ(3) since 3 generates the group of units modulo 4.

The Dirichlet *L*-series for is
the Dirichlet lambda function (closely related to the Dirichlet eta function)

where is the Riemann zeta-function. The *L*-series for is the Dirichlet beta-function

### Modulus 5Edit

There are characters modulo 5. In the table below, *i* is the imaginary unit.

χ \ *n***0****1****2****3****4**0 1 1 1 1 0 1 *i*− *i*−1 0 1 −1 −1 1 0 1 − *i**i*−1

Note that χ is wholly determined by χ(2) and χ(3) since 2 and 3 generate the group of units modulo 5.

### Modulus 6Edit

There are characters modulo 6:

χ \ *n***0****1****2****3****4****5**0 1 0 0 0 1 0 1 0 0 0 −1

Note that χ is wholly determined by χ(5) since 5 generates the group of units modulo 6.

### Modulus 7Edit

There are characters modulo 7. In the table below,

χ \ *n***0****1****2****3****4****5****6**0 1 1 1 1 1 1 0 1 ω ^{2}ω ω ω ^{2}1 0 1 ω ω ^{2}ω ^{2}ω 1 0 1 1 −1 1 −1 −1 0 1 ω ^{2}−ω ω −ω ^{2}−1 0 1 ω −ω ^{2}ω ^{2}−ω −1

Note that χ is wholly determined by χ(3) since 3 generates the group of units modulo 7.

### Modulus 8Edit

There are characters modulo 8.

χ \ *n***0****1****2****3****4****5****6****7**0 1 0 1 0 1 0 1 0 1 0 1 0 −1 0 −1 0 1 0 −1 0 1 0 −1 0 1 0 −1 0 −1 0 1

Note that χ is wholly determined by χ(3) and χ(5) since 3 and 5 generate the group of units modulo 8.

### Modulus 9Edit

There are characters modulo 9. In the table below,

χ \ *n***0****1****2****3****4****5****6****7****8**0 1 1 0 1 1 0 1 1 0 1 ω 0 ω ^{2}−ω ^{2}0 −ω −1 0 1 ω ^{2}0 −ω −ω 0 ω ^{2}1 0 1 −1 0 1 −1 0 1 −1 0 1 −ω 0 ω ^{2}ω ^{2}0 −ω 1 0 1 −ω ^{2}0 −ω ω 0 ω ^{2}−1

Note that χ is wholly determined by χ(2) since 2 generates the group of units modulo 9.

### Modulus 10Edit

There are characters modulo 10. In the table below, *i* is the imaginary unit.

χ \ *n***0****1****2****3****4****5****6****7****8****9**0 1 0 1 0 0 0 1 0 1 0 1 0 *i*0 0 0 − *i*0 −1 0 1 0 −1 0 0 0 −1 0 1 0 1 0 − *i*0 0 0 *i*0 −1

Note that χ is wholly determined by χ(3) since 3 generates the group of units modulo 10.

### Modulus 11Edit

There are characters modulo 11. In the table below,

χ \ *n***0****1****2****3****4****5****6****7****8****9****10**0 1 1 1 1 1 1 1 1 1 1 0 1 ω ω ^{3}ω ^{2}ω ^{4}ω ^{4}ω ^{2}ω ^{3}ω 1 0 1 ω ^{2}ω ω ^{4}ω ^{3}ω ^{3}ω ^{4}ω ω ^{2}1 0 1 ω ^{3}ω ^{4}ω ω ^{2}ω ^{2}ω ω ^{4}ω ^{3}1 0 1 ω ^{4}ω ^{2}ω ^{3}ω ω ω ^{3}ω ^{2}ω ^{4}1 0 1 −1 1 1 1 −1 −1 −1 1 −1 0 1 −ω ω ^{3}ω ^{2}ω ^{4}−ω ^{4}−ω ^{2}−ω ^{3}ω −1 0 1 −ω ^{2}ω ω ^{4}ω ^{3}−ω ^{3}−ω ^{4}−ω ω ^{2}−1 0 1 −ω ^{3}ω ^{4}ω ω ^{2}−ω ^{2}−ω −ω ^{4}ω ^{3}−1 0 1 −ω ^{4}ω ^{2}ω ^{3}ω −ω −ω ^{3}−ω ^{2}ω ^{4}−1

Note that χ is wholly determined by χ(2) since 2 generates the group of units modulo 11.

### Modulus 12Edit

There are characters modulo 12.

χ \ *n***0****1****2****3****4****5****6****7****8****9****10****11**0 1 0 0 0 1 0 1 0 0 0 1 0 1 0 0 0 1 0 −1 0 0 0 −1 0 1 0 0 0 −1 0 1 0 0 0 −1 0 1 0 0 0 −1 0 −1 0 0 0 1

Note that χ is wholly determined by χ(5) and χ(7) since 5 and 7 generate the group of units modulo 12.

## ExamplesEdit

If *p* is an odd prime number, then the function

- where is the Legendre symbol, is a primitive Dirichlet character modulo
*p*.^{[12]}

More generally, if *m* is a positive odd number, the function

- where is the Jacobi symbol, is a Dirichlet character modulo
*m*.^{[12]}

These are examples of real characters. In general, all real characters arise from the Kronecker symbol.

## Primitive characters and conductorEdit

Residues mod *N* give rise to residues mod *M*, for any factor *M* of *N*, by discarding some information. The effect on Dirichlet characters goes in the opposite direction: if χ is a character mod *M*, it *induces* a character χ* mod *N* for any multiple *N* of *M*. A character is **primitive** if it is not induced by any character of smaller modulus.^{[3]}

If χ is a character mod *n* and *d* divides *n*, then we say that the modulus *d* is an *induced modulus* for χ if *a* coprime to *n* and 1 mod *d* implies χ(*a*)=1:^{[13]} equivalently, χ(*a*) = χ(*b*) whenever *a*, *b* are congruent mod *d* and each coprime to *n*.^{[14]} A character is primitive if there is no smaller induced modulus.^{[14]}

We can formalize this differently by defining characters χ_{1} mod *N*_{1} and χ_{2} mod *N*_{2} to be **co-trained** if for some modulus *N* such that *N*_{1} and *N*_{2} both divide *N* we have χ_{1}(*n*) = χ_{2}(*n*) for all *n* coprime to *N*: that is, there is some character χ* induced by each of χ_{1} and χ_{2}. In that case, there exists a character modulo the gcd of *N*_{1} and *N*_{2} inducing both χ_{1} and χ_{2.} This is an equivalence relation on characters. A character with the smallest modulus, in the sense of divisibility, in an equivalence class is primitive and this smallest modulus is the **conductor** of the characters in the class.

Imprimitivity of characters can lead to missing Euler factors in their L-functions.

## Character orthogonalityEdit

The orthogonality relations for characters of a finite group transfer to Dirichlet characters.^{[15]} If we fix a character χ modulo *n* then the sum

unless χ is principal, in which case the sum is φ(*n*). Similarly, if we fix a residue class *a* modulo *n* and sum over all characters we have

unless in which case the sum is φ(*n*). We deduce that any periodic function with period *n* supported on the residue classes prime to *n* is a linear combination of Dirichlet characters.^{[16]} We also have the a character sum relation given in Chapter 4 of Davenport given by

where the sum is taken over all Dirichlet characters modulo some fixed q, *a* and *n* are fixed with , and denotes Euler's totient function.

## HistoryEdit

Dirichlet characters and their *L*-series were introduced by Peter Gustav Lejeune Dirichlet, in 1831, in order to prove Dirichlet's theorem on arithmetic progressions. He only studied the *L*-series for real *s* and especially as *s* tends to 1. The extension of these functions to complex *s* in the whole complex plane was obtained by Bernhard Riemann in 1859.

## See alsoEdit

## ReferencesEdit

**^**Montgomery & Vaughan (2007) pp.117–8**^**Montgomery & Vaughan (2007) p.115- ^
^{a}^{b}Montgomery & Vaughan (2007) p.123 **^**Fröhlich & Taylor (1991) p.218**^**Frohlich & Taylor (1991) p.215**^**Apostol (1976) p.139- ^
^{a}^{b}^{c}Apostol (1976) p.138 **^**Apostol (1976) p.134**^**Sarkozy, Andras. "On multiplicative arithmetic functions satisfying a linear recursion".*Studia Sci. Math. Hung*.**13**(1–2): 79–104.**^**Chudakov, N.G. "Theory of the characters of number semigroups".*J. Indian Math. Soc*.**20**: 11–15.**^**Klurman, Oleksiy; Mangerel, Alexander P. (2017). "Rigidity Theorems for Multiplicative Functions".*Math. Ann*.**372**(1): 651–697. arXiv:1707.07817. Bibcode:2017arXiv170707817K. doi:10.1007/s00208-018-1724-6.- ^
^{a}^{b}Montgomery & Vaughan (2007) p.295 **^**Apostol (1976) p.166- ^
^{a}^{b}Apostol (1976) p.168 **^**Apostol (1976) p.140**^**Davenport (1967) pp.31–32

- See chapter 6 of Apostol, Tom M. (1976),
*Introduction to analytic number theory*, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001 - Apostol, T. M. (1971). "Some properties of completely multiplicative arithmetical functions".
*The American Mathematical Monthly*.**78**(3): 266–271. doi:10.2307/2317522. JSTOR 2317522. MR 0279053. Zbl 0209.34302. - Davenport, Harold (1967).
*Multiplicative number theory*. Lectures in advanced mathematics.**1**. Chicago: Markham. Zbl 0159.06303. - Hasse, Helmut (1964).
*Vorlesungen über Zahlentheorie*. Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen.**59**(2nd revised ed.). Springer-Verlag. MR 0188128. Zbl 0123.04201. see chapter 13. - Mathar, R. J. (2010). "Table of Dirichlet L-series and prime zeta modulo functions for small moduli". arXiv:1008.2547 [math.NT].
- Montgomery, Hugh L; Vaughan, Robert C. (2007).
*Multiplicative number theory. I. Classical theory*. Cambridge Studies in Advanced Mathematics.**97**. Cambridge University Press. ISBN 978-0-521-84903-6. Zbl 1142.11001. - Spira, Robert (1969). "Calculation of Dirichlet L-Functions".
*Mathematics of Computation*.**23**(107): 489–497. doi:10.1090/S0025-5718-1969-0247742-X. MR 0247742. Zbl 0182.07001. - Fröhlich, A.; Taylor, M.J. (1991).
*Algebraic number theory*. Cambridge studies in advanced mathematics.**27**. Cambridge University Press. ISBN 0-521-36664-X. Zbl 0744.11001.

## External linksEdit

- "Dirichlet character",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - "Dirichlet Characters". in the LMFDB