# Drinfel'd module

A Drinfel'd -module, (where is an appropriate ring) over a field of characteristic is an exotic -module structure on the additive group over . In several regards, the concept of a Drinfel'd module is analogous to the concept of an elliptic curve (or more generally, of an irreducible Abelian variety), with which it shares many features. Among the similarities between Drinfel'd modules and elliptic curves are the respective structures of torsion points, of Tate modules and of endomorphism rings, the existence of analytic "Weierstrass uniformizations" , and the moduli theories (modular varieties, modular forms; cf. also Modular form). Many topics from the (classical and well-developed) theory of elliptic curves may be transferred to Drinfel'd modules, thereby revealing arithmetical information about the ground field . On the other hand, since the mechanism of Drinfel'd modules is smoother and in some respects simpler than that of Abelian varieties, some results involving Drinfel'd modules over global function fields can be proved, whose analogues over number fields are far from being settled (e.g. parts of Stark's conjectures, of the Langlands conjectures, assertions about the arithmetical nature of zeta values and other questions of transcendence theory over , cf. also -function). The invention and basic theory as well as large parts of the deeper results about Drinfel'd modules are due to V.G. Drinfel'd [a3], [a4]. General references are [a2], [a10], [a9], and [a8].

## Algebraic theory.

Let be any field of characteristic , with algebraic closure . The endomorphism ring of the additive group scheme is the ring of additive polynomials , i.e., of polynomials satisfying , whose (non-commutative) multiplication is defined by insertion. Then

is the twisted polynomial ring in with commutation rule for and unit element . Fix a power of . If contains the field with elements, one sets and for the subring of -linear polynomials in . For any -algebra , an -module structure on is given by a morphism of -algebras from to .

Fix a (smooth, projective, geometrically connected) algebraic curve over and a place "∞" of ; let be its function field and the affine ring of . (Here, "places" , or "primes" , are closed points of , the set of normalized valuations on ; cf. also Norm on a field.) Hence is a function field in one variable over and is its subring of elements regular away from . Put for the associated degree function: if . Let be a field equipped with a structure of an -algebra. Then is either an extension of or of some , where is a maximal ideal. One writes in the former and in the latter case. A Drinfel'd -module of rank over (briefly, an -Drinfel'd module over ) is a morphism of -algebras

subject to:

i) ; and

ii) for . It supplies the additive group of each -algebra with the structure of an abstract -module. A morphism of Drinfel'd -modules over is some element of that satisfies for . Similarly, one defines iso-, endo- and automorphisms.

The standard example of is given by a rational function field , being the usual place at infinity, . In that case, a Drinfel'd -module of rank over is given by , where the may be arbitrarily chosen in (). More generally, if is generated over by , is given by the that in must satisfy the same relations as do the in . Writing down a Drinfel'd module amounts to solving a complicated system of polynomial equations over . For example, let with . From , , one obtains for a -Drinfel'd module over : with , . Using computation rules in , one solves for , , , , which yields the unique (up to isomorphism) -Drinfel'd module over . That is unique and even definable over corresponds to the fact that the class number of equals .

Let with and let be an -Drinfel'd module over . The polynomial has degree in , whence has different roots in if . This implies that the -module of -torsion points of is isomorphic with . Similar, but more complicated assertions hold if one considers -torsion points ( a not necessarily principal ideal of ) and if divides . A level- structure on is the choice of an isomorphism of abstract -modules (with some modification if divides ).

The definitions of Drinfel'd modules, their morphisms, torsion points, and level structures generalize to arbitrary -schemes (instead of , which corresponds to the case above; cf. also Scheme). Intuitively, an -Drinfel'd module over is a family of -Drinfel'd modules varying continuously over . Let be a non-vanishing ideal. On the category of -schemes , there is the contravariant functor that to each associates the set of isomorphism classes of -Drinfel'd modules over provided with a level- structure. If has at least two prime divisors (such are admissible), is representable by a moduli scheme . In other words, -morphisms from to correspond one-to-one to isomorphism classes of -Drinfel'd modules over with a level- structure. The various and are equipped with actions of the finite groups and related by morphisms if . Taking quotients, this allows one to define coarse moduli schemes even for non-admissible ideals , and for more general moduli problems, e.g., the problem "rank-r Drinfel'd A-modules with a point of order a A" . For such coarse moduli schemes, the above bijection between morphisms from to and objects of the moduli problem holds only if is the spectrum of an algebraically closed field. If is admissible, is affine, smooth, of finite type and of dimension over . Furthermore, for , the morphisms from to are finite and flat, and even étale outside the support of (cf. also Affine morphism; Flat morphism; Etale morphism). As an example, take , and let be algebraically closed. Two -Drinfel'd modules and over , given through the coefficients and () of and , are isomorphic if and only if there exists a such that . Hence , the moduli scheme attached to the trivial ideal of , is the open subscheme defined by of , where the multiplicative group acts diagonally through . If , with the "modular invariant" , the being regarded as indeterminates.

## Analytic theory.

Let be the completion of at , with normalized absolute value and complete algebraic closure . Then is the smallest field extension of which is complete with respect to and algebraically closed. For such fields, there is a reasonable function theory and analytic geometry [a1].

An -lattice is a finitely generated (thus projective) -submodule of that has finite intersection with each ball in . With is associated its exponential function , defined as the everywhere convergent infinite product (). It is a surjective, -linear and -periodic function that for each satisfies a functional equation with some . The rule defines a ring homomorphism from to , in fact, a Drinfel'd -module of rank , being the projective rank of the -module . Each -Drinfel'd module over is so obtained, and yields an equivalence of the category of lattices of projective rank with the category of -Drinfel'd modules over . (A morphism of lattices is some such that .) The description of through the lattice is called the Weierstrass uniformization. From , one can read off many of the properties of . E.g.,

and (). For , there result bijections between the finite sets of:

a) classes of rank-one -lattices in , up to scalars;

b) ideal classes of , i.e., ;

c) isomorphism classes of rank-one Drinfel'd modules over , i.e., . For , let be the analytic subspace

of . Note that , which is the Drinfel'd upper half-plane. The set (in fact, -analytic space) of -valued points of the moduli scheme may now be described as a finite union of quotients of by subgroups of commensurable with , in much the same way as one usually describes the moduli of elliptic curves over . In the standard example , , , one obtains the -analytic isomorphisms , where . The left-hand mapping associates with the Drinfel'd module with , and the right-hand mapping is given by the modular invariant . Writing , the coefficients and become functions in , in fact, modular forms for of respective weights and . Moduli problems with non-trivial level structures correspond to subgroups of , i.e., to modular curves , which are ramified covers of the above. As "classically" these curves may be studied function-theoretically via the modular forms for . The same holds, more or less, for more general base rings than and for higher ranks than . Quite generally, the moduli schemes encode essential parts of the arithmetic of and , as will be demonstrated by the examples below.

## Applications.

### Explicit Abelian class field theory of .

Adjoining torsion points of rank-one Drinfel'd modules results in Abelian extensions of the base field. Applying this to the -Drinfel'd module defined by (the so-called Carlitz module) yields all the Abelian extensions of that are tamely ramified at , similar to cyclotomic extensions of the field of rationals. This also works for general base rings with class numbers ; here the situation resembles the theory of complex multiplication of elliptic curves [a11] (cf. also Elliptic curve).

### Langlands conjectures in characteristic .

The moduli scheme is equipped with an action of (where is the ring of finite adèles of ). It is a major problem to determine the representation type of the -adic cohomology modules , i.e., to express them in terms of automorphic representations. This can partially be achieved and leads to (local or global) reciprocity laws between representations of and Galois representations (cf. also Galois theory). In particular, the local Langlands correspondence for over a local field of equal characteristic may so be proved [a4], [a13], [a12].

### Modularity conjecture over .

As a special case of the previous subsection, the Galois representations associated to elliptic curves over may be found in . This leads to a Shimura–Taniyama–Weil correspondence between elliptic curves over with split multiplicative reduction at , isogeny factors of dimension one of Jacobians of certain Drinfel'd modular curves and (effectively calculable) automorphic Hecke eigenforms over [a7].

### Cohomology of arithmetic groups.

Invariants like Betti numbers, numbers of cusps, Euler–Poincaré-characteristics of subgroups of are related to the geometry of the moduli scheme . In some cases (e.g., ), these invariants may be determined using the theory of Drinfel'd modular forms [a5].

### Arithmetic of division algebras.

Exploiting the structure of endomorphism rings of Drinfel'd modules over finite -fields and using knowledge of the moduli schemes, one can find formulas for class and type numbers of central division algebras over [a6].

## Curves with many rational points.

Drinfel'd modules provide explicit constructions of algebraic curves over finite fields with predictable properties. In particular, curves with many rational points compared to their genera may be tailored [a14].

Other features and deep results in the field that definitely should be mentioned are the following:

the transcendence theory of Drinfel'd modules, their periods, and special values of exponential lattice functions, mainly created by J. Yu [a17];

D. Goss has developed a theory of -valued zeta- and -functions for Drinfel'd modules and similar objects [a9];

R. Pink has proved an analogue of the Tate conjecture (cf. also Tate conjectures) for Drinfel'd modules [a15];

H.-G. Rück and U. Tipp have proved a Gross–Zagier-type formula for heights of Heegner points on Drinfel'd modular curves [a16].

#### References

[a1] | S. Bosch, U. Güntzer, R. Remmert, "Non-Archimedean analysis" , Springer (1984) |

[a2] | P. Deligne, D. Husemöller, "Survey of Drinfel'd modules" Contemp. Math. , 67 (1987) pp. 25–91 |

[a3] | V.G. Drinfel'd, "Elliptic modules" Math. USSR Sb. , 23 (1976) pp. 561–592 |

[a4] | V.G. Drinfel'd, "Elliptic modules II" Math. USSR Sb. , 31 (1977) pp. 159–170 |

[a5] | E.-U. Gekeler, "Drinfeld modular curves" , Lecture Notes Math. , 1231 , Springer (1986) |

[a6] | E.-U. Gekeler, "On the arithmetic of some division algebras" Comment. Math. Helvetici , 67 (1992) pp. 316–333 |

[a7] | E.-U. Gekeler, M. Reversat, "Jacobians of Drinfeld modular curves" J. Reine Angew. Math. , 476 (1996) pp. 27–93 |

[a8] | "Drinfeld modules, modular schemes and applications" E.-U. Gekeler (ed.) M. van der Put (ed.) M. Reversat (ed.) J. van Geel (ed.) , World Sci. (1997) |

[a9] | D. Goss, "Basic structures of function field arithmetic" , Springer (1996) |

[a10] | "The arithmetic of function fields" D. Goss (ed.) D. Hayes (ed.) M. Rosen (ed.) , W. de Gruyter (1992) |

[a11] | D. Hayes, "Explicit class field theory in global function fields" , Studies Algebra and Number Th. , Adv. Math. , 16 (1980) pp. 173–217 |

[a12] | G. Laumon, "Cohomology of Drinfeld modular varieties I,II" , Cambridge Univ. Press (1996/7) |

[a13] | G. Laumon, M. Rapoport, U. Stuhler, "-elliptic sheaves and the Langlands correspondence" Invent. Math. , 113 (1993) pp. 217–338 |

[a14] | H. Niederreiter, C. Xing, "Cyclotomic function fields, Hilbert class fields, and global function fields with many rational places" Acta Arith. , 79 (1997) pp. 59–76 |

[a15] | R. Pink, "The Mumford–Tate conjecture for Drinfeld modules" Publ. RIMS Kyoto Univ. , 33 (1997) pp. 393–425 |

[a16] | H.-G. Rück, U. Tipp, "Heegner points and L-series of automorphic cusp forms of Drinfeld type" Preprint Essen (1998) |

[a17] | J. Yu, "Transcendence and Drinfeld modules" Invent. Math. , 83 (1986) pp. 507–517 |

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Drinfel'd module.

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