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A complete [[algebraic curve]]  $  X _ {\widetilde \Gamma   } $
+
A complete [[Algebraic curve|algebraic curve]]  $  X _ {\widetilde \Gamma } $
 
uniformized by a subgroup  $  \widetilde \Gamma  $
 
uniformized by a subgroup  $  \widetilde \Gamma  $
 
of finite index in the [[Modular group|modular group]]  $  \Gamma $;  
 
of finite index in the [[Modular group|modular group]]  $  \Gamma $;  
Line 20: Line 20:
 
The best known examples of subgroups  $  \widetilde \Gamma  $
 
The best known examples of subgroups  $  \widetilde \Gamma  $
 
of finite index in  $  \Gamma $
 
of finite index in  $  \Gamma $
are the [[congruence subgroup]]s containing a principal congruence subgroup  $  \Gamma ( N) $
+
are the congruence subgroups containing a principal congruence subgroup  $  \Gamma (N) $
 
of level  $  N $
 
of level  $  N $
 
for some integer  $  N > 1 $,  
 
for some integer  $  N > 1 $,  
 
represented by the matrices
 
represented by the matrices
  
$$  
+
$$ A  \in {\mathop{\rm SL}\nolimits} _ {2} ( \mathbf Z ) ,\ A  \equiv \left ( \begin{array}{cc}  1  & 0  \\  0  & 1  \\ \end{array}  \right ) \mathop{\rm mod}  N $$
A  \in   \mathrm{SL} _ {2} ( \mathbf Z ) ,\ \
 
A  \equiv \
 
\left (  
 
\begin{array}{cc}
 
  1  & 0  \\
 
  0  & 1  \\
 
\end{array}
 
  \right )   \mathop{\rm mod}  N
 
$$
 
  
(see [[Modular group]]). The least such  $  N $
+
(see [[Modular group|Modular group]]). The least such  $  N $
 
is called the level of the subgroup  $  \widetilde \Gamma  $.  
 
is called the level of the subgroup  $  \widetilde \Gamma  $.  
In particular, the subgroup  $  \Gamma _ {0} ( N) $
+
In particular, the subgroup  $  \Gamma _ {0} (N) $
represented by matrices which are congruent  $   \mathop{\rm mod}  N $
+
represented by matrices which are congruent  $ {\mathop{\rm mod}\nolimits}  N $
 
to upper-triangular matrices has level  $  N $.  
 
to upper-triangular matrices has level  $  N $.  
 
Corresponding to each subgroup  $  \widetilde \Gamma  $
 
Corresponding to each subgroup  $  \widetilde \Gamma  $
of finite index there is a covering of the modular curve  $  X _ {\widetilde \Gamma   } \rightarrow X _  \Gamma  $,  
+
of finite index there is a covering of the modular curve  $  X _ {\widetilde \Gamma } \mathop \rightarrow \limits X _  \Gamma  $,  
 
which ramifies only over the images of the points  $  z = i $,  
 
which ramifies only over the images of the points  $  z = i $,  
 
$  z = ( 1 + i \sqrt 3 ) / 2 $,  
 
$  z = ( 1 + i \sqrt 3 ) / 2 $,  
$  z = \infty $.  
+
$  z = \inf $.  
 
For a congruence subgroup  $  \widetilde \Gamma  $
 
For a congruence subgroup  $  \widetilde \Gamma  $
the ramification of this covering allows one to determine the genus of  $  X _ {\widetilde \Gamma   } $
+
the ramification of this covering allows one to determine the genus of  $  X _ {\widetilde \Gamma } $
 
and to prove the existence of subgroups  $  \widetilde \Gamma  $
 
and to prove the existence of subgroups  $  \widetilde \Gamma  $
 
of finite index in  $  \Gamma $
 
of finite index in  $  \Gamma $
which are not congruence subgroups (see [[#References|[4]]], Vol. 2, [[#References|[2]]]). The genus of  $  X _ {\Gamma ( N) } $
+
which are not congruence subgroups (see [[#References|[4]]], Vol. 2, [[#References|[2]]]). The genus of  $  X _ {\Gamma (N)} $
 
is  $  0 $
 
is  $  0 $
 
for  $  N \leq  2 $
 
for  $  N \leq  2 $
 
and equals
 
and equals
  
$$  
+
$$ 1 + \frac{N ^ {2} ( N - 6 )}{24} \prod _ {p \mid  N} ( 1 - p ^ {-2} ) , $$
1 +
 
 
 
\frac{N ^ {2} ( N - 6 ) }{24}
 
 
 
\prod _ {p \mid  N }
 
( 1 - p ^ {-2} ) ,
 
$$
 
  
 
$  p $
 
$  p $
 
a prime number, for  $  N > 2 $.  
 
a prime number, for  $  N > 2 $.  
 
A modular curve is always defined over an algebraic number field (usually over  $  \mathbf Q $
 
A modular curve is always defined over an algebraic number field (usually over  $  \mathbf Q $
or a cyclic extension of it). The rational functions on a modular curve lift to modular functions (of a higher level) and form a field; the automorphisms of this field have been studied (see [[#References|[2]]]). A holomorphic differential form on a modular curve  $  X _ {\widetilde \Gamma   } $
+
or a cyclic extension of it). The rational functions on a modular curve lift to modular functions (of a higher level) and form a field; the automorphisms of this field have been studied (see [[#References|[2]]]). A holomorphic differential form on a modular curve  $  X _ {\widetilde \Gamma } $
 
is given on  $  H $
 
is given on  $  H $
by a differential  $  f ( z)  d z $(
+
by a differential  $  f (z)  d z $  
where  $  f ( z) $
+
(where  $  f (z) $
is a holomorphic function) which is invariant under the transformations  $  z \rightarrow \gamma ( z) $
+
is a holomorphic function) which is invariant under the transformations  $  z \mathop \rightarrow \limits \gamma (z) $
 
of  $  \widetilde \Gamma  $;  
 
of  $  \widetilde \Gamma  $;  
here  $  f ( z) $
+
here  $  f (z) $
 
is a cusp form of weight 2 relative to  $  \widetilde \Gamma  $.  
 
is a cusp form of weight 2 relative to  $  \widetilde \Gamma  $.  
The [[Zeta-function|zeta-function]] of a modular curve is a product of the Mellin transforms (cf. [[Mellin transform|Mellin transform]]) of modular forms and, consequently, has a meromorphic continuation and satisfies a functional equation. This fact serves as the point of departure for the Langlands–Weil theory on the relationship between modular forms and Dirichlet series (see [[#References|[7]]], [[#References|[8]]]). In particular, there is a hypothesis that each [[Elliptic curve|elliptic curve]] over  $  \mathbf Q $(
+
The [[Zeta-function|zeta-function]] of a modular curve is a product of the Mellin transforms (cf. [[Mellin transform|Mellin transform]]) of modular forms and, consequently, has a meromorphic continuation and satisfies a functional equation. This fact serves as the point of departure for the Langlands–Weil theory on the relationship between modular forms and Dirichlet series (see [[#References|[7]]], [[#References|[8]]]). In particular, there is a hypothesis that each [[Elliptic curve|elliptic curve]] over  $  \mathbf Q $  
with conductor  $  N $)  
+
(with conductor  $  N $)  
 
can be uniformized by modular functions of level  $  N $.  
 
can be uniformized by modular functions of level  $  N $.  
 
The homology of a modular curve is connected with modular symbols, which allows one to investigate the arithmetic of the values of the zeta-function of a modular curve in the centre of the critical strip and to construct the  $  p $-
 
The homology of a modular curve is connected with modular symbols, which allows one to investigate the arithmetic of the values of the zeta-function of a modular curve in the centre of the critical strip and to construct the  $  p $-
 
adic zeta-function of a modular curve (see [[#References|[1]]]).
 
adic zeta-function of a modular curve (see [[#References|[1]]]).
  
A modular curve parametrizes a family of elliptic curves, being their moduli variety (see [[#References|[7]]], Vol. 2). In particular, for  $  \widetilde \Gamma  = \Gamma ( N) $
+
A modular curve parametrizes a family of elliptic curves, being their moduli variety (see [[#References|[7]]], Vol. 2). In particular, for  $  \widetilde \Gamma  = \Gamma (N) $
 
a point  $  z $
 
a point  $  z $
of  $  H / \Gamma ( N) $
+
of  $  H / \Gamma (N) $
is in one-to-one correspondence with a pair consisting of an elliptic curve  $  E _ {z} $(
+
is in one-to-one correspondence with a pair consisting of an elliptic curve  $  E _ {z} $  
analytically equivalent to a complex torus  $  \mathbf C / ( \mathbf Z + \mathbf Z z) $)  
+
(analytically equivalent to a complex torus  $  \mathbf C / ( \mathbf Z + \mathbf Z z) $)  
 
and a point of order  $  N $
 
and a point of order  $  N $
on  $  E _ {z} $(
+
on  $  E _ {z} $  
the image of  $  z / N $).
+
(the image of  $  z / N $).
  
Over each modular curve  $  X _ {\widetilde \Gamma   } $
+
Over each modular curve  $  X _ {\widetilde \Gamma } $
there is a natural algebraic fibre bundle  $  E _ {\widetilde \Gamma   } \rightarrow X _ {\widetilde \Gamma   } $
+
there is a natural algebraic fibre bundle  $  E _ {\widetilde \Gamma } \mathop \rightarrow \limits X _ {\widetilde \Gamma } $
 
of elliptic curves if  $  \widetilde \Gamma  $
 
of elliptic curves if  $  \widetilde \Gamma  $
 
does not contain  $  - 1 $,  
 
does not contain  $  - 1 $,  
compactified by degenerate curves above the parabolic points of  $  X _ {\widetilde \Gamma   } $.  
+
compactified by degenerate curves above the parabolic points of  $  X _ {\widetilde \Gamma } $.  
Powers  $  E _ { {\widetilde \Gamma   } } ^ {( w) } $,  
+
Powers  $  E _ { {\widetilde \Gamma }} ^ {(w)} $,  
 
where  $  w \geq  1 $
 
where  $  w \geq  1 $
is an integer, are called Kuga varieties (see [[#References|[3]]], [[#References|[5]]]). The zeta- functions of  $  E _ {{\widetilde \Gamma   } } ^ {( w) } $
+
is an integer, are called Kuga varieties (see [[#References|[3]]], [[#References|[5]]]). The zeta- functions of  $  E _ { {\widetilde \Gamma }} ^ {(w)} $
 
are related to the Mellin transforms of modular forms, and their homology to the periods of modular forms (see [[#References|[3]]], [[#References|[7]]]).
 
are related to the Mellin transforms of modular forms, and their homology to the periods of modular forms (see [[#References|[3]]], [[#References|[7]]]).
  
 
The rational points on a modular curve correspond to elliptic curves having rational points of finite order (or rational subgroups of points); their description (see [[#References|[6]]]) made it possible to solve the problem of determining the possible torsion subgroups of elliptic curves over  $  \mathbf Q $.
 
The rational points on a modular curve correspond to elliptic curves having rational points of finite order (or rational subgroups of points); their description (see [[#References|[6]]]) made it possible to solve the problem of determining the possible torsion subgroups of elliptic curves over  $  \mathbf Q $.
  
The investigation of the geometry and arithmetic of modular curves is based on the use of groups of automorphisms of the projective limit of the curves  $  X _ {\widetilde \Gamma   } $
+
The investigation of the geometry and arithmetic of modular curves is based on the use of groups of automorphisms of the projective limit of the curves  $  X _ {\widetilde \Gamma } $
 
with respect to decreasing  $  \widetilde \Gamma  $,  
 
with respect to decreasing  $  \widetilde \Gamma  $,  
which (in essence) coincides with the group  $   \mathrm{ SL} _ {2} ( A) $
+
which (in essence) coincides with the group  $ {\mathop{\rm SL}\nolimits} _ {2} (A) $
 
over the ring  $  A $
 
over the ring  $  A $
of rational adèles. On each modular curve  $  X _ {\widetilde \Gamma   } $
+
of rational adèles. On each modular curve  $  X _ {\widetilde \Gamma } $
this gives a non-trivial ring of correspondences  $  R _ {\widetilde \Gamma   } $(
+
this gives a non-trivial ring of correspondences  $  R _ {\widetilde \Gamma } $  
a Hecke ring), which has applications in the theory of modular forms (cf. [[Modular form|Modular form]], [[#References|[3]]]).
+
(a Hecke ring), which has applications in the theory of modular forms (cf. [[Modular form|Modular form]], [[#References|[3]]]).
  
 
====References====
 
====References====

Latest revision as of 11:16, 21 June 2020


A complete algebraic curve $ X _ {\widetilde \Gamma } $ uniformized by a subgroup $ \widetilde \Gamma $ of finite index in the modular group $ \Gamma $; more precisely, a modular curve is a complete algebraic curve obtained from a quotient space $ H / \widetilde \Gamma $, where $ H $ is the upper half-plane, together with a finite number of parabolic points (the equivalence classes relative to $ \widetilde \Gamma $ of the rational points of the boundary of $ H $). The best known examples of subgroups $ \widetilde \Gamma $ of finite index in $ \Gamma $ are the congruence subgroups containing a principal congruence subgroup $ \Gamma (N) $ of level $ N $ for some integer $ N > 1 $, represented by the matrices

$$ A \in {\mathop{\rm SL}\nolimits} _ {2} ( \mathbf Z ) ,\ A \equiv \left ( \begin{array}{cc} 1 & 0 \\ 0 & 1 \\ \end{array} \right ) \mathop{\rm mod} N $$

(see Modular group). The least such $ N $ is called the level of the subgroup $ \widetilde \Gamma $. In particular, the subgroup $ \Gamma _ {0} (N) $ represented by matrices which are congruent $ {\mathop{\rm mod}\nolimits} N $ to upper-triangular matrices has level $ N $. Corresponding to each subgroup $ \widetilde \Gamma $ of finite index there is a covering of the modular curve $ X _ {\widetilde \Gamma } \mathop \rightarrow \limits X _ \Gamma $, which ramifies only over the images of the points $ z = i $, $ z = ( 1 + i \sqrt 3 ) / 2 $, $ z = \inf $. For a congruence subgroup $ \widetilde \Gamma $ the ramification of this covering allows one to determine the genus of $ X _ {\widetilde \Gamma } $ and to prove the existence of subgroups $ \widetilde \Gamma $ of finite index in $ \Gamma $ which are not congruence subgroups (see [4], Vol. 2, [2]). The genus of $ X _ {\Gamma (N)} $ is $ 0 $ for $ N \leq 2 $ and equals

$$ 1 + \frac{N ^ {2} ( N - 6 )}{24} \prod _ {p \mid N} ( 1 - p ^ {-2} ) , $$

$ p $ a prime number, for $ N > 2 $. A modular curve is always defined over an algebraic number field (usually over $ \mathbf Q $ or a cyclic extension of it). The rational functions on a modular curve lift to modular functions (of a higher level) and form a field; the automorphisms of this field have been studied (see [2]). A holomorphic differential form on a modular curve $ X _ {\widetilde \Gamma } $ is given on $ H $ by a differential $ f (z) d z $ (where $ f (z) $ is a holomorphic function) which is invariant under the transformations $ z \mathop \rightarrow \limits \gamma (z) $ of $ \widetilde \Gamma $; here $ f (z) $ is a cusp form of weight 2 relative to $ \widetilde \Gamma $. The zeta-function of a modular curve is a product of the Mellin transforms (cf. Mellin transform) of modular forms and, consequently, has a meromorphic continuation and satisfies a functional equation. This fact serves as the point of departure for the Langlands–Weil theory on the relationship between modular forms and Dirichlet series (see [7], [8]). In particular, there is a hypothesis that each elliptic curve over $ \mathbf Q $ (with conductor $ N $) can be uniformized by modular functions of level $ N $. The homology of a modular curve is connected with modular symbols, which allows one to investigate the arithmetic of the values of the zeta-function of a modular curve in the centre of the critical strip and to construct the $ p $- adic zeta-function of a modular curve (see [1]).

A modular curve parametrizes a family of elliptic curves, being their moduli variety (see [7], Vol. 2). In particular, for $ \widetilde \Gamma = \Gamma (N) $ a point $ z $ of $ H / \Gamma (N) $ is in one-to-one correspondence with a pair consisting of an elliptic curve $ E _ {z} $ (analytically equivalent to a complex torus $ \mathbf C / ( \mathbf Z + \mathbf Z z) $) and a point of order $ N $ on $ E _ {z} $ (the image of $ z / N $).

Over each modular curve $ X _ {\widetilde \Gamma } $ there is a natural algebraic fibre bundle $ E _ {\widetilde \Gamma } \mathop \rightarrow \limits X _ {\widetilde \Gamma } $ of elliptic curves if $ \widetilde \Gamma $ does not contain $ - 1 $, compactified by degenerate curves above the parabolic points of $ X _ {\widetilde \Gamma } $. Powers $ E _ { {\widetilde \Gamma }} ^ {(w)} $, where $ w \geq 1 $ is an integer, are called Kuga varieties (see [3], [5]). The zeta- functions of $ E _ { {\widetilde \Gamma }} ^ {(w)} $ are related to the Mellin transforms of modular forms, and their homology to the periods of modular forms (see [3], [7]).

The rational points on a modular curve correspond to elliptic curves having rational points of finite order (or rational subgroups of points); their description (see [6]) made it possible to solve the problem of determining the possible torsion subgroups of elliptic curves over $ \mathbf Q $.

The investigation of the geometry and arithmetic of modular curves is based on the use of groups of automorphisms of the projective limit of the curves $ X _ {\widetilde \Gamma } $ with respect to decreasing $ \widetilde \Gamma $, which (in essence) coincides with the group $ {\mathop{\rm SL}\nolimits} _ {2} (A) $ over the ring $ A $ of rational adèles. On each modular curve $ X _ {\widetilde \Gamma } $ this gives a non-trivial ring of correspondences $ R _ {\widetilde \Gamma } $ (a Hecke ring), which has applications in the theory of modular forms (cf. Modular form, [3]).

References

[1] Yu.I. Manin, "Parabolic points and zeta-functions of modular curves" Math. USSR Izv. , 6 : 1 (1972) pp. 19–64 Izv. Akad. Nauk SSSR Ser. Mat. , 36 : 1 (1972) pp. 19–66
[2] G. Shimura, "Introduction to the arithmetic theory of automorphic functions" , Math. Soc. Japan (1971)
[3] V.V. [V.V. Shokurov] Šokurov, "Holomorphic differential forms of higher degree on Kuga's modular varieties" Math. USSR Sb. , 30 : 1 (1976) pp. 119–142 Mat. Sb. , 101 : 1 (1976) pp. 131–157
[4] F. Klein, R. Fricke, "Vorlesungen über die Theorie der elliptischen Modulfunktionen" , 1–2 , Teubner (1890–1892)
[5] M. Kuga, G. Shimura, "On the zeta function of a fibre variety whose fibres are abelian varieties" Ann. of Math. , 82 (1965) pp. 478–539
[6] B. Mazur, J.-P. Serre, "Points rationnels des courbes modulaires (d'après A. Ogg)" , Sem. Bourbaki 1974/1975 , Lect. notes in math. , 514 , Springer (1976) pp. 238–255
[7] J.-P. Serre (ed.) P. Deligne (ed.) W. Kuyk (ed.) , Modular functions of one variable. 1–6 , Lect. notes in math. , 320; 349; 350; 476; 601; 627 , Springer (1973–1977)
[8] A. Weil, "Ueber die Bestimmung Dirichletscher Reihen durch Funktionalgleichungen" Math. Ann. , 168 (1967) pp. 149–156

Comments

References

[a1] N.M. Katz, B. Mazur, "Arithmetic moduli of elliptic curves" , Princeton Univ. Press (1985)
How to Cite This Entry:
Modular curve. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Modular_curve&oldid=49734
This article was adapted from an original article by A.A. PanchishkinA.N. Parshin (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article