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| In [[number theory]] an '''ideal number''' is an [[algebraic integer]] which represents an [[ideal (ring theory)|ideal]] in the [[ring (mathematics)|ring]] of integers of a [[number field]]; the idea was developed by [[Ernst Kummer]], and led to [[Richard Dedekind]]'s definition of [[ideal (ring theory)|ideal]]s for rings. An ideal in the ring of integers of an algebraic number field is '''principal''' if it consists of multiples of a single element of the ring, and '''nonprincipal''' otherwise. By the [[principal ideal theorem]] any nonprincipal ideal becomes principal when extended to an ideal of the [[Hilbert class field]]. This means that there is an element of the ring of integers of the Hilbert class field, which is an ideal number, such that the original nonprincipal ideal is equal to the collection of all multiples of this ideal number by elements of this [[ring of integers]] that lie in the original field's ring of integers.
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| ==Example==
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| For instance, let ''y'' be a root of ''y''<sup>2</sup> + ''y'' + 6 = 0, then the ring of integers of the field <math>\Bbb{Q}(y)</math> is <math>\Bbb{Z}[y]</math>, which means all ''a'' + ''by'' with ''a'' and ''b'' integers form the ring of integers. An example of a nonprincipal ideal in this ring is 2''a'' + ''yb'' with ''a'' and ''b'' integers; the cube of this ideal is principal, and in fact the [[class group]] is cyclic of order three. The corresponding class field is obtained by adjoining an element ''w'' satisfying ''w''<sup>3</sup> − ''w'' − 1 = 0 to <math>\Bbb{Q}(y)</math>, giving <math>\Bbb{Q}(y,w)</math>. An ideal number for the nonprincipal ideal 2''a'' + ''yb'' is <math>\iota = (-8-16y-18w+12w^2+10yw+yw^2)/23</math>. Since this satisfies the equation
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| <math>\iota^6-2\iota^5+13\iota^4-15\iota^3+16\iota^2+28\iota+8 = 0</math> it is an algebraic integer.
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| All elements of the ring of integers of the class field which when multiplied by ι give a result in <math>\Bbb{Z}[y]</math> are of the form ''a''α + ''b''β, where
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| :<math>\alpha = (-7+9y-33w-24w^2+3yw-2yw^2)/23\ </math>
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| and
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| :<math>\beta = (-27-8y-9w+6w^2-18yw-11yw^2)/23.\ </math>
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| The coefficients α and β are also algebraic integers, satisfying
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| :<math>\alpha^6+7\alpha^5+8\alpha^4-15\alpha^3+26\alpha^2-8\alpha+8=0\,</math>
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| and
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| :<math>\beta^6+4\beta^5+35\beta^4+112\beta^3+162\beta^2+108\beta+27=0\,</math>
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| respectively. Multiplying ''a''α + ''b''β by the ideal number ι gives 2''a'' + ''by'', which is the nonprincipal ideal.
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| ==History==
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| Kummer first published the failure of unique factorization in [[cyclotomic fields]] in 1844 in an obscure journal; it was reprinted in 1847 in [[Joseph Liouville|Liouville's]] journal. In subsequent papers in 1846 and 1847 he published his main theorem, the unique factorization into (actual and ideal) primes.
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| It is widely believed that Kummer was led to his "ideal complex numbers" by his interest in [[Fermat's Last Theorem]]; there is even a story often told that Kummer, like [[Gabriel Lamé|Lamé]], believed he had proven Fermat's Last Theorem until [[Peter Gustav Lejeune Dirichlet|Lejeune Dirichlet]] told him his argument relied on unique factorization; but the story was first told by [[Kurt Hensel]] in 1910 and the evidence indicates it likely derives from a confusion by one of Hensel's sources. [[Harold Edwards (mathematician)|Harold Edwards]] says the belief that Kummer was mainly interested in Fermat's Last Theorem "is surely mistaken" (op cit p. 79). Kummer's use of the letter λ to represent a prime number, α to denote a λth root of unity, and his study of the factorization of prime number <math>p\equiv 1 \pmod{\lambda}</math> into "complex numbers composed of <math>\lambda</math>th roots of unity" all derive directly from a paper of [[Carl Gustav Jakob Jacobi|Jacobi]] which is concerned with [[higher reciprocity laws]]. Kummer's 1844 memoir was in honor of the jubilee celebration of the University of Königsberg and was meant as a tribute to Jacobi. Although Kummer had studied Fermat's Last Theorem in the 1830s and was probably aware that his theory would have implications for its study, it is more likely that the subject of Jacobi's (and [[Carl Friedrich Gauss|Gauss's]]) interest, higher reciprocity laws, held more importance for him. Kummer referred to his own partial proof of Fermat's Last Theorem for [[regular prime]]s as "a curiosity of number theory rather than a major item" and to the higher reciprocity law (which he stated as a conjecture) as "the principal subject and the pinnacle of contemporary number theory." On the other hand, this latter pronouncement was made when Kummer was still excited about the success of his work on reciprocity and when his work on Fermat's Last Theorem was running out of steam, so it may perhaps be taken with some skepticism.
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| The extension of Kummer's ideas to the general case was accomplished independently by Kronecker and Dedekind during the next forty years. A direct generalization encountered formidable difficulties, and it eventually led Dedekind to the creation of the theory of [[module (mathematics)|modules]] and [[ideal (ring theory)|ideals]]. Kronecker dealt with the difficulties by developing a theory of forms (a generalization of [[quadratic forms]]) and a theory of [[divisor (algebraic geometry)|divisors]]. Dedekind's contribution would become the basis of [[ring theory]] and [[abstract algebra]], while Kronecker's would become major tools in [[algebraic geometry]].
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| ==References==
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| *[[Nicolas Bourbaki]], ''Elements of the History of Mathematics.'' Springer-Verlag, NY, 1999.
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| *[[Harold M. Edwards]], ''Fermat's Last Theorem. A genetic introduction to number theory.'' Graduate Texts in Mathematics vol. 50, Springer-Verlag, NY, 1977.
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| *C.G. Jacobi, ''Über die complexen Primzahlen, welche in der theori der Reste der 5ten, 8ten, und 12ten Potenzen zu betrachten sind,'' Monatsber. der. Akad. Wiss. Berlin (1839) 89-91.
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| *E.E. Kummer, ''De numeris complexis, qui radicibus unitatis et numeris integris realibus constant,'' Gratulationschrift der Univ. Breslau zur Jubelfeier der Univ. Königsberg, 1844; reprinted in ''Jour. de Math.'' 12 (1847) 185-212.
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| *E.E. Kummer, ''Über die Zerlegung der aus Wurzeln der Einheit gebildeten complexen Zahlen in ihre Primfactoren,'' Jour. für Math. (Crelle) 35 (1847) 327-367.
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| *John Stillwell, introduction to ''Theory of Algebraic Integers'' by Richard Dedekind. Cambridge Mathematical Library, Cambridge University Press, Great Britain, 1996.
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| ==External links==
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| * [http://fermatslasttheorem.blogspot.com/2006/07/cyclotomic-integers-ideal-numbers_25.html Ideal Numbers], Proof that the theory of ideal numbers saves unique factorization for cyclotomic integers at [http://fermatslasttheorem.blogspot.com Fermat's Last Theorem Blog].
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| [[Category:Number theory]]
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| [[Category:Numbers]]
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