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{{Short description|Plane algebraic curve}}
In [[number theory]], the '''classical modular curve''' is an irreducible [[algebraic curve|plane algebraic curve]] given by an equation
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such that {{math|(''x'', ''y'') {{=}} (''j''(''nτ''), ''j''(''τ''))}} is a point on the curve. Here {{math|''j''(''τ'')}} denotes the [[j-invariant|{{mvar|j}}-invariant]].
The curve is sometimes called {{math|''X''<sub>0</sub>(''n'')}}, though often that notation is used for the abstract [[algebraic curve]] for which there exist various models. A related object is the '''classical modular polynomial''', a polynomial in one variable defined as {{math|Φ<sub>''n''</sub>(''x'', ''x'')}}.
== Geometry of the modular curve ==
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and factor, we get an extraneous factor of a rational function of {{mvar|x}}, and the curve {{math|''y''<sup>2</sup> + ''y'' {{=}} ''x''<sup>3</sup> − ''x''<sup>2</sup>}}, with {{mvar|j}}-invariant {{math|−2<sup>12</sup>11<sup>−1</sup>}}. Hence both curves are modular of level {{math|11}}, having mappings from {{math|''X''<sub>0</sub>(11)}}.
By a theorem of [[Henri Carayol]], if an elliptic curve {{mvar|E}} is modular then its [[conductor of an elliptic curve|conductor]], an isogeny invariant described originally in terms of [[cohomology]], is the smallest integer {{mvar|n}} such that there exists a rational mapping {{math|''φ'' : ''X''<sub>0</sub>(''n'') → ''E''}}. Since we now know all elliptic curves over {{math|'''Q'''}} are modular, we also know that the conductor is simply the level {{mvar|n}} of its minimal modular parametrization.
== Galois theory of the modular curve ==
The [[Galois theory]] of the modular curve was investigated by [[Erich Hecke]]. Considered as a polynomial in x with coefficients in {{math|'''Z'''[''y'']}}, the modular equation {{math|Φ<sub>0</sub>(''n'')}} is a polynomial of degree {{math|''ψ''(''n'')}} in {{mvar|x}}, whose roots generate a [[Galois extension]] of {{math|'''Q'''(''y'')}}. In the case of {{math|''X''<sub>0</sub>(''p'')}} with {{mvar|p}} prime, where the [[Characteristic (algebra)|characteristic]] of the field is not {{mvar|p}}, the [[Galois group]] of {{math|'''Q'''(''x'', ''y'')/'''Q'''(''y'')}} is {{math|PGL(2, ''p'')}}, the [[projective linear group|projective general linear group]] of [[Möbius transformation|linear fractional transformations]] of the [[projective line]] of the field of {{mvar|p}} elements, which has {{math|''p'' + 1}} points, the degree of {{math|''X''<sub>0</sub>(''p'')}}.
This extension contains an algebraic extension {{math|''F''/'''Q'''}} where if <math>p^* = (-1)^{(p-1)/2}p</math> in the notation of [[
:<math>F = \mathbf{Q}\left(\sqrt{p^*}\right).</math>
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== References ==
*{{citation|first=Erich
*Anthony Knapp, ''Elliptic Curves'', Princeton, 1992
*[[Serge Lang]], ''Elliptic Functions'', Addison-Wesley, 1973
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