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| In [[mathematics]], '''Dirichlet's unit theorem''' is a basic result in [[algebraic number theory]] due to [[Peter Gustav Lejeune Dirichlet]].<ref>{{harvnb|Elstrodt|2007|loc=§8.D}}</ref> It determines the [[rank of an abelian group|rank]] of the [[group of units]] in the [[ring (mathematics)|ring]] ''O''<sub>''K''</sub> of [[algebraic integer]]s of a [[number field]] ''K''. The '''regulator''' is a positive real number that determines how "dense" the units are.
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| ==Dirichlet's unit theorem==
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| The statement is that the group of units is finitely generated and has [[Rank of an abelian group|rank]] (maximal number of multiplicatively independent elements) equal to
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| :''r'' = ''r''<sub>1</sub> + ''r''<sub>2</sub> − 1
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| where ''r''<sub>1</sub> is the ''number of real embeddings'' and ''r''<sub>2</sub> the ''number of conjugate pairs of complex embeddings'' of ''K''. This characterisation of
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| ''r''<sub>1</sub> and ''r''<sub>2</sub> is based on the idea that there will be as many ways to embed ''K'' in the [[complex number]] field as the degree ''n'' = [''K'' : '''Q''']; these will either be into the [[real number]]s, or pairs of embeddings related by [[complex conjugation]], so that
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| :''n'' = ''r''<sub>1</sub> + 2''r''<sub>2</sub>.
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| Note that if ''K'' is Galois over ''Q'' then either ''r''<sub>1</sub> is non-zero or ''r''<sub>2</sub> is non-zero, but not both.
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| Other ways of determining ''r''<sub>1</sub> and ''r''<sub>2</sub> are
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| * use the [[Primitive element (field theory)|primitive element]] theorem to write ''K'' = '''Q'''(α), and then ''r''<sub>1</sub> is the number of [[conjugate element (field theory)|conjugates]] of α that are real, 2''r''<sub>2</sub> the number that are complex;
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| * write the [[tensor product of fields]] ''K'' ⊗<sub>'''Q'''</sub>'''R''' as a product of fields, there being ''r''<sub>1</sub> copies of '''R''' and ''r''<sub>2</sub> copies of '''C'''.
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| As an example, if ''K'' is a [[quadratic field]], the rank is 1 if it is a real quadratic field, and 0 if an imaginary quadratic field. The theory for real quadratic fields is essentially the theory of [[Pell's equation]].
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| The rank is > 0 for all number fields besides '''Q''' and imaginary quadratic fields, which have rank 0. The 'size' of the units is measured in general by a [[determinant]] called the regulator. In principle a basis for the units can be effectively computed; in practice the calculations are quite involved when ''n'' is large.
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| The torsion in the group of units is the set of all roots of unity of ''K'', which form a finite [[cyclic group]]. For a number field with at least one real embedding the torsion
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| must therefore be only {1,−1}. There are number fields, for example most [[imaginary quadratic field]]s, having no real embeddings which also have {1,−1} for the torsion of its unit group.
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| Totally real fields are special with respect to units. If ''L/K'' is a finite extension of number fields with degree greater than 1 and
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| the units groups for the integers of ''L'' and ''K'' have the same rank then ''K'' is totally real and ''L'' is a totally complex quadratic extension. The converse
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| holds too. (An example is
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| ''K'' equal to the rationals and ''L'' equal to an imaginary quadratic field; both have unit rank 0.)
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| There is a generalisation of the unit theorem by [[Helmut Hasse]] (and later [[Claude Chevalley]]) to describe the structure of the group of ''[[S-unit]]s'', determining the rank of the unit group in [[localization of a ring|localizations]] of rings of integers. Also, the [[Galois module]] structure of <math>\mathbf{Q} \oplus O_{K,S} \otimes_\mathbf{Z} \mathbf{Q}</math> has been determined.{{sfn|Neukirch|Schmidt|Wingberg|2000|loc=proposition VIII.8.6.11}}
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| ==The regulator==
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| Suppose that ''u''<sub>1</sub>,...,''u''<sub>r</sub> are a set of generators for the unit group modulo roots of unity. If ''u'' is an algebraic number, write ''u''<sup>1</sup>, ..., ''u''<sup>''r+1''</sup> for the different embeddings into '''R''' or '''C''', and set
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| ''N''<sub>''j''</sub> to 1, resp. 2 if corresponding embedding is real, resp. complex.
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| Then the ''r'' by ''r'' + 1 matrix whose entries are <math>N_j\log|u_i^j|</math> has the property that the sum of any row is zero (because all units have norm 1, and the log of the norm is the sum of the entries of a row). This implies that the absolute value ''R'' of the determinant of the submatrix formed by deleting one column is independent of the column.
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| The number ''R'' is called the '''regulator''' of the algebraic number field (it does not depend on the choice of generators ''u''<sub>i</sub>). It measures the "density" of the units: if the regulator is small, this means that there are "lots" of units.
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| The regulator has the following geometric interpretation. The map taking a unit ''u'' to the vector with entries <math>N_j\log|u^j|</math> has image in the ''r''-dimensional subspace of '''R'''<sup>''r''+1</sup> consisting
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| of all vector whose entries have sum 0, and by Dirichlet's unit theorem the image is a lattice in this subspace. The volume of a fundamental domain of this lattice is ''R''√(''r''+1).
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| The regulator of an algebraic number field of degree greater than 2 is usually quite cumbersome to calculate, though there are now computer algebra packages that can do it in many cases. It is usually much easier to calculate the product ''hR'' of the [[class number (number theory)|class number]] ''h'' and the regulator using the [[class number formula]], and the main difficulty in calculating the class number of an algebraic number field is usually the calculation of the regulator.
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| ===Examples===
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| [[Image:Discriminant49CubicFieldFundamentalDomainOfUnits.png|thumb|300px|right|A fundamental domain in logarithmic space of the group of units of the cyclic cubic field ''K'' obtained by adjoining to '''Q''' a root of ''f''(''x'') = ''x''<sup>3</sup> + ''x''<sup>2</sup> − 2''x'' − 1. If α denotes a root of ''f''(''x''), then a set of fundamental units is {ε<sub>1</sub>, ε<sub>2</sub>} where ε<sub>1</sub> = α<sup>2</sup> + α − 1 and ε<sub>2</sub> = 2 − α<sup>2</sup>. The area of the fundamental domain is approximately 0.910114, so the regulator of ''K'' is approximately 0.525455.]]
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| *The regulator of an [[imaginary quadratic field]], or of the rational integers, is 1 (as the determinant of a 0×0 matrix is 1).
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| *The regulator of a [[real quadratic field]] is the logarithm of its [[fundamental unit]]: for example, that of '''Q'''(√5) is log((√5 + 1)/2). This can be seen as follows. A fundamental unit is (√5 + 1)/2, and its images under the two embeddings into '''R''' are (√5 + 1)/2 and (−√5 + 1)/2. So the ''r'' by ''r'' + 1 matrix is
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| ::<math>\left[1\times\log\left|{\sqrt{5} + 1 \over 2}\right|, \quad 1\times \log\left|{-\sqrt{5} + 1 \over 2}\right|\ \right].</math>
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| *The regulator of the [[cyclic cubic field]] '''Q'''(α), where α is a root of ''x''<sup>3</sup> + ''x''<sup>2</sup> − 2''x'' − 1, is approximately 0.5255. A basis of the group of units modulo roots of unity is {ε<sub>1</sub>, ε<sub>2</sub>} where ε<sub>1</sub> = α<sup>2</sup> + α − 1 and ε<sub>2</sub> = 2 − α<sup>2</sup>.<ref>{{harvnb|Cohen|1993|loc=Table B.4}}</ref>
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| ==Higher regulators==
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| A 'higher' regulator refers to a construction for a function on an [[algebraic K-group]] with index ''n'' > 1 that plays the same role as the classical regulator does for the group of units, which is a group ''K''<sub>1</sub>. A theory of such regulators has been in development, with work of [[Armand Borel]] and others. Such higher regulators play a role, for example, in the [[Beilinson conjectures]], and are expected to occur in evaluations of certain [[L-function]]s at integer values of the argument.<ref name=Bloch>{{cite book | last=Bloch | first=Spencer J. | authorlink=Spencer Bloch | title=Higher regulators, algebraic ''K''-theory, and zeta functions of elliptic curves | series=CRM Monograph Series | volume=11 | location=Providence, RI | publisher=[[American Mathematical Society]] | year=2000 | isbn=0-8218-2114-8 | zbl=0958.19001 }}</ref>
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| ==Stark regulator==
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| The formulation of [[Stark's conjectures]] led [[Harold Stark]] to define what is now called the '''Stark regulator''', similar to the classical regulator as a determinant of logarithms of units, attached to any [[Artin representation]].<ref>[http://www.math.tifr.res.in/~dprasad/artin.pdf PDF]</ref><ref>[http://www.math.harvard.edu/~dasgupta/papers/Dasguptaseniorthesis.pdf PDF]</ref>
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| ==''p''-adic regulator==
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| Let ''K'' be a [[number field]] and for each [[Valuation (algebra)|prime]] ''P'' of ''K'' above some fixed rational prime ''p'', let ''U''<sub>''P''</sub> denote the local units at ''P'' and let ''U''<sub>1,''P''</sub> denote the subgroup of principal units in ''U''<sub>''P''</sub>. Set
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| : <math> U_1 = \prod_{P|p} U_{1,P}. </math>
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| Then let ''E''<sub>1</sub> denote the set of global units ''ε'' that map to ''U''<sub>1</sub> via the diagonal embedding of the global units in ''E''.
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| Since <math>E_1</math> is a finite-[[Index of a subgroup|index]] subgroup of the global units, it is an [[abelian group]] of rank <math>r_1 + r_2 - 1</math>. The '''''p''-adic regulator''' is the determinant of the matrix formed by the ''p''-adic logarithms of the generators of this group. ''[[Leopoldt's conjecture]]'' states that this determinant is non-zero.<ref name=NSW6267>Neukirch et al (2008) p.626-627</ref><ref>{{cite book | last=Iwasawa | first=Kenkichi | authorlink=Kenkichi Iwasawa | title=Lectures on ''p''-adic ''L''-functions | series=Annals of Mathematics Studies | volume=74 | location=Princeton, NJ | publisher=Princeton University Press and University of Tokyo Press | year=1972 | isbn=0-691-08112-3 | zbl=0236.12001 | pages=36-42 }}</ref>
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| ==See also==
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| *[[Elliptic unit]]
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| *[[Cyclotomic unit]]
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| *[[Shintani's unit theorem]]
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| * {{Cite book
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| | last=Cohen
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| | first=Henri
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| | author-link=Henri Cohen (number theorist)
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| | title=A Course in Computational Algebraic Number Theory
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| | publisher=[[Springer-Verlag]]
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| | location=Berlin, New York
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| | series=[[Graduate Texts in Mathematics]] | volume=138
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| | isbn=978-3-540-55640-4
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| | mr=1228206 | zbl=0786.11071
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| | year=1993
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| | ref=harv
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| }}
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| *{{cite journal | last = Elstrodt | first = Jürgen | journal = Clay Mathematics Proceedings
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| | title = The Life and Work of Gustav Lejeune Dirichlet (1805–1859) | work = | publisher = | year = 2007
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| | url = http://www.uni-math.gwdg.de/tschinkel/gauss-dirichlet/elstrodt-new.pdf | format = PDF | doi =
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| | accessdate = 2010-06-13 | ref = harv}}
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| * {{cite book | first=Serge | last=Lang | author-link=Serge Lang | title=Algebraic number theory | edition=2nd | series=Graduate Texts in Mathematics | volume=110 | location=New York | publisher=[[Springer-Verlag]] | year=1994 | isbn=0-387-94225-4 | zbl=0811.11001 }}
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| *{{Neukirch ANT}}
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| *{{Neukirch et al. CNF}}
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| [[Category:Theorems in algebraic number theory]]
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