Hecke character explained

In number theory, a Hecke character is a generalisation of a Dirichlet character, introduced by Erich Hecke to construct a class ofL-functions larger than Dirichlet L-functions, and a natural setting for the Dedekind zeta-functions and certain others which have functional equations analogous to that of the Riemann zeta-function.

Definition

A Hecke character is a character of the idele class group of a number field or global function field. It corresponds uniquely to a character of the idele group which is trivial on principal ideles, via composition with the projection map.

This definition depends on the definition of a character, which varies slightly between authors: It may be defined as a homomorphism to the non-zero complex numbers (also called a "quasicharacter"), or as a homomorphism to the unit circle in

("unitary"). Any quasicharacter (of the idele class group) can be written uniquely as a unitary character times a real power of the norm, so there is no big difference between the two definitions.

The conductor of a Hecke character

\chi

is the largest ideal

ak{m}

such that

\chi

is a Hecke character mod

ak{m}

. Here we say that

\chi

is a Hecke character mod

ak{m}

\chi

(considered as a character on the idele group) is trivial on the group of finite ideles whose every

\nu

-adic component lies in

1+ak{m}O_\nu

Größencharakter

, let

ak{m}=ak{m}_fak{m}_infty

be a

-modulus, with

ak{m}_f

, the "finite part", being an integral ideal of

and

ak{m}_infty

, the "infinite part", being a (formal) product of real places of

. Let

I_ak{m}

denote the group of fractional ideals of

relatively prime to

ak{m}_f

andlet

P_ak{m}

denote the subgroup of principal fractional ideals

(a)

where

is near

at each place of

ak{m}

in accordance with the multiplicities ofits factors. That is, for each finite place

\nu

ak{m}_f

, the order

ord_\nu(a-1)

is at least as large as the exponent for

\nu

ak{m}_f

, and

is positive under each real embedding in

ak{m}_infty

. A Größencharakter with modulus

ak{m}

is a group homomorphism from

I_ak{m}

into the nonzero complex numbers such that on ideals

(a)

P_ak{m}

its value is equal to thevalue at

of a continuous homomorphism to the nonzero complex numbers from the product of the multiplicative groups of all Archimedean completions of

where each local component of the homomorphism has the same real part (in the exponent). (Here we embed

into the product of Archimedean completions of

using embeddings corresponding to the various Archimedean places on

.) Thus a Größencharakter may be defined on the ray class group modulo

ak{m}

, which is the quotient

I_ak{m}/P_ak{m}

Strictly speaking, Hecke made the stipulation about behavior on principal ideals for those admitting a totally positive generator. So, in terms of the definition given above, he really only worked with moduli where all real places appeared.The role of the infinite part m_∞ is now subsumed under the notion ofan infinity-type.

Relationship between Größencharakter and Hecke character

A Hecke character and a Größencharakter are essentially the same notion with a one-to-one correspondence. The ideal definition is much more complicated than the idelic one, and Hecke's motivation for his definition was to construct L-functions (sometimes referred to as Hecke L-functions)^[1] that extend the notion of a Dirichlet L-function from the rationals to other number fields. For a Größencharakter χ, its L-function is defined to be the Dirichlet series

\sum_(I,m)=1\chi(I)N(I)^-s=L(s,\chi)

carried out over integral ideals relatively prime to the modulus

ak{m}

of the Größencharakter.Here

N(I)

denotes the ideal norm. The common real part condition governing the behavior of Größencharakter on the subgroups

P_ak{m}

implies theseDirichlet series are absolutely convergent in some right half-plane. Hecke proved these L-functions have a meromorphic continuation to the whole complex plane, being analytic except for a simple pole of order 1 at '

s=1

when the character is trivial. For primitive Größencharakter (defined relative to a modulus in a similar manner to primitive Dirichlet characters), Hecke showed these L-functions satisfy a functional equation relating the values of the L-function of a character and the L-function of its complex conjugate character.

Consider a character

\psi

of the idele class group, taken to be a map into the unit circle which is 1 on principal ideles and on an exceptional finite set

containing all infinite places. Then

\psi

generates a character

\chi

of the ideal group

I^S

, which is the free abelian group on the prime ideals not in

.^[2] Take a uniformising element

\pi

for each prime

ak{p}

not in

and define a map

\Pi

from

I^S

to idele classes by mapping each

ak{p}

to the class of the idele which is

\pi

in the

ak{p}

coordinate and

everywhere else. Let

\chi

be the composite of

\Pi

and

\psi

. Then

\chi

is well-defined as a character on the ideal group.^[3]

In the opposite direction, given an admissible character

\chi

I^S

there corresponds a unique idele class character

\psi

.^[4] Here admissible refers to the existence of a modulus

ak{m}

based on the set

such that the character

\chi

evaluates to

on the ideals which are 1 mod

ak{m}

.^[5]

The characters are 'big' in the sense that the infinity-type when present non-trivially means these characters are not of finite order. The finite-order Hecke characters are all, in a sense, accounted for by class field theory: their L-functions are Artin L-functions, as Artin reciprocity shows. But even a field as simple as the Gaussian field has Hecke characters that go beyond finite order in a serious way (see the example below). Later developments in complex multiplication theory indicated that the proper place of the 'big' characters was to provide the Hasse–Weil L-functions for an important class of algebraic varieties (or even motives).

Special cases

A Dirichlet character is a Hecke character of finite order. It is determined by values on the set of totally positive principal ideals which are 1 with respect to some modulus m.^[5]
A Hilbert character is a Dirichlet character of conductor 1.^[5] The number of Hilbert characters is the order of the class group of the field. Class field theory identifies the Hilbert characters with the characters of the Galois group of the Hilbert class field.

Examples

R⁺

with all the unit groups of the p-adic integers. So a quasicharacter can be written as product of a power of the norm with a Dirichlet character.

A Hecke character χ of the Gaussian integers of conductor 1 is of the form

χ((a)) = |a|^s(a/|a|)⁴ⁿ

for s imaginary and n an integer, where a is a generator of the ideal (a). The only units are powers of i, so the factor of 4 in the exponent ensures that the character is well defined on ideals.

Tate's thesis

See main article: Tate's thesis. Hecke's original proof of the functional equation for L(s,χ) used an explicit theta-function. John Tate's 1950 Princeton doctoral dissertation, written under the supervision of Emil Artin, applied Pontryagin duality systematically, to remove the need for any special functions. A similar theory was independently developed by Kenkichi Iwasawa which was the subject of his 1950 ICM talk. A later reformulation in a Bourbaki seminar by showed that parts of Tate's proof could be expressed by distribution theory: the space of distributions (for Schwartz–Bruhat test functions) on the adele group of K transforming under the action of the ideles by a given χ has dimension 1.

Algebraic Hecke characters

An algebraic Hecke character is a Hecke character taking algebraic values: they were introduced by Weil in 1947 under the name type A₀. Such characters occur in class field theory and the theory of complex multiplication.^[6]

Indeed let E be an elliptic curve defined over a number field F with complex multiplication by the imaginary quadratic field K, and suppose that K is contained in F. Then there is an algebraic Hecke character χ for F, with exceptional set S the set of primes of bad reduction of E together with the infinite places. This character has the property that for a prime ideal p of good reduction, the value χ(p) is a root of the characteristic polynomial of the Frobenius endomorphism. As a consequence, the Hasse–Weil zeta function for E is a product of two Dirichlet series, for χ and its complex conjugate.^[7]

References

Book: J.W.S. . Cassels . J. W. S. Cassels . Albrecht . Fröhlich . Albrecht Fröhlich . Algebraic Number Theory . 1967 . Academic Press . 0153.07403 .
Book: Heilbronn, H. . Hans Heilbronn . VIII. Zeta-functions and L-functions . 204–230 . J.W.S. . Cassels . J. W. S. Cassels . Albrecht . Fröhlich . Albrecht Fröhlich . Algebraic Number Theory . 1967 . Academic Press .
Book: Husemöller, Dale H. . Elliptic curves . With an appendix by Ruth Lawrence . Graduate Texts in Mathematics . 111 . . 1987 . 0-387-96371-5 . 0605.14032 .
Book: Husemöller, Dale . Elliptic curves . 2002 . second . . . 111 . 0-387-95490-2 . 10.1007/b97292 . 1040.11043 .
Book: W. Narkiewicz . Elementary and analytic theory of algebraic numbers . registration . 2nd . . 1990 . 3-540-51250-0 . 334–343 . 0717.11045 .
- J. Tate, Fourier analysis in number fields and Hecke's zeta functions (Tate's 1950 thesis), reprinted in Algebraic Number Theory edd J. W. S. Cassels, A. Fröhlich (1967) pp. 305–347.
Book: Tate, J.T. . John Tate (mathematician) . VII. Global class field theory . 162–203 . J.W.S. . Cassels . J. W. S. Cassels . Albrecht . Fröhlich . Albrecht Fröhlich . Algebraic Number Theory . 1967 . Academic Press . 1179.11041 .

Notes and References

As in
Heilbronn (1967) p.204
Heilbronn (1967) p. 205
Tate (1967) p.169
Heilbronn (1967) p.207
Husemoller (1987) pp. 299–300; (2002) p.320
Husemoller (1987) pp. 302–303; (2002) pp. 321–322