View source for Siegel theorem

Siegel's theorem on Dirichlet <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850102.png" />-functions: For any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850103.png" /> there exists a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850104.png" /> such that for any non-principal real [[Dirichlet character|Dirichlet character]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850105.png" /> of modulus <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850106.png" />,

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First proved by C.L. Siegel [[#References|[1]]]. An equivalent formulation is concerned with real zeros of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s0850109.png" />-functions: For any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501010.png" /> there exists a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501011.png" /> such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501012.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501013.png" /> for any non-principal real Dirichlet character <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501014.png" />. The constants <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501015.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501016.png" /> are non-effective, in the sense that for no <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501017.png" /> there is a way to estimate them from below. For this reason, applications of Siegel's theorem have a non-constructive character. For example, if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501018.png" /> is the divisor class number of a quadratic field of discriminant <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501019.png" />, it follows from the theorem that

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with a non-effective constant <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501021.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501022.png" />. Similarly, the following estimate, which is uniform for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501023.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501024.png" />,

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where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501026.png" /> is the number of primes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501027.png" /> representable as <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501028.png" />, involves a non-effective constant <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501029.png" /> (cf. also [[Distribution of prime numbers|Distribution of prime numbers]]).

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  C.L. Siegel,   "Ueber die Klassenzahl quadratischen Zahlkorper"  ''Acta Arithmetica'' , '''1'''  (1935)  pp. 83–86</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  H. Davenport,   "Multiplicative number theory" , Springer  (1981)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  A.A. Karatsuba,   "Fundamentals of analytic number theory" , Moscow  (1975)  pp. Chapt. 9  (In Russian)</TD></TR></table>

''S.M. Voronin''

Siegel's theorem on integral points is a theorem stating that Diophantine equations of a certain class have finitely many integral solutions. In its simplest form, the theorem states that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501030.png" /> is a polynomial with integer coefficients, defining an irreducible curve <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501031.png" /> of genus <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501032.png" />, then the equation <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501033.png" /> has finitely many integer solutions. This result (actually in a more general form, for algebraic integers) was established in 1929 by C.L. Siegel [[#References|[1]]], who used the theories of Diophantine approximations and Abelian varieties. It was the culmination of a line of research in the theory of Diophantine equations initiated in 1908 by A. Thue (see [[#References|[2]]], [[#References|[3]]]). The theorem was subsequently generalized to the case of arbitrary affine curves of genus <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501034.png" /> defined over subrings of finite type in global fields (see [[#References|[5]]]). In particular, the above equation <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501035.png" /> has finitely many solutions of the form

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where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501037.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501038.png" /> are fixed prime numbers. Till recently, the proofs of this theorem suffered from a common drawback — while stating that the number of solutions is finite, they provided no quantitative estimates (see [[Height, in Diophantine geometry|Height, in Diophantine geometry]]), so that one had no algorithm for an explicit construction of solutions. The first result free of this shortcoming was due to A. Baker (1967). Effective proofs of Siegel's theorem have been obtained for various classes of Diophantine equations, but in the general case the problem is still open (see [[#References|[4]]]). Another important problem is to generalize Siegel's theorem to varieties of dimension greater than one.

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  C.L. Siegel,   "Ueber einige Anwendungen diophantischer Approximationen"  ''Abh. Preuss. Akad. Wiss. Phys.-Math. Kl.'' , '''1'''  (1929)  pp. 41–69</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  A.O. Gel'fond,   "The solution of equations in integers" , Noordhoff  (1960)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  H. Davenport,   "The higher arithmetic" , Hutchinson  (1952)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top"> , ''Problems in the theory of Diophantine approximations'' , Moscow  (1974)  (In Russian; translated from English)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  S. Lang,   "Diophantine geometry" , Interscience  (1962)</TD></TR></table>

''A.N. Parshin''

====Comments====
The Mordell conjecture states that for any number field and any smooth curve <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501039.png" /> of genus <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501040.png" /> defined over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501041.png" /> there are only finitely many <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s085/s085010/s08501042.png" />-rational solutions. This was proved by G. Faltings in 1983.

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  B. Mazur,   "On some of the mathematical contributions of Gerd Faltings" , ''Proc. Internat. Congress Mathematicians (Berkeley, 1986)'' , Amer. Math. Soc.  (1987)  pp. 7–12</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  G. Faltings,   "Endlichkeitssätze für abelschen Varietäten über Zahlkörper"  ''Invent. Math.'' , '''73'''  (1983)  pp. 349–366</TD></TR></table>