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| {{Algebraic structures |Ring}}
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| In [[mathematics]], and more specifically in [[algebra]], a '''domain''' is a [[ring (mathematics)|ring]] such that ''ab'' = 0 implies that either ''a'' = 0 or ''b'' = 0.<ref>Polcino M. & Sehgal (2002), p. 65.</ref> That is, it is a ring which has no left or right [[zero divisor]]s. (Sometimes such a ring is said to "have the [[zero-product property]].") Some authors require the ring to be [[trivial ring|nontrivial]] (that is, it must have more than one element).<ref name="Lanski">Lanski (2005), p. 343, Definition 10.18.</ref> If the domain has a [[multiplicative identity]] (which we may call 1), this is equivalent to saying that 1 ≠ 0<ref>Jacobson (2009), p. 90, Section 2.2. Note that if 1=0, then a=1a=0a=0 showing that all elements are 0.</ref> Thus a domain is a nontrivial ring without left or right zero divisors. A [[commutative ring|commutative]] domain with 1 ≠ 0 is called an [[integral domain]].<ref>Rowen (1994), p. 99.</ref>
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| A ''finite'' domain is automatically a [[finite field]] by [[Wedderburn's little theorem]].
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| Zero divisors have a topological interpretation, at least in the case of commutative rings: a ring ''R'' is an integral domain, if and only if it is [[reduced ring|reduced]] and its [[Spectrum of a ring|spectrum]] ''Spec R'' is an [[irreducible topological space]]. The first property is often considered to encode some infinitesimal information, whereas the second one is more geometric.
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| An example: the ring ''k''[''x'', ''y'']/(''xy''), where ''k'' is a field, is not a domain, as the images of ''x'' and ''y'' in this ring are zero divisors. Geometrically, this corresponds to the fact that the spectrum of this ring, which is the union of the lines ''x'' = 0 and ''y'' = 0, is not irreducible. Indeed, these two lines are its irreducible components.
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| == Constructions of domains ==
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| One way of proving that a ring is a domain is by exhibiting a filtration with special properties.
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| '''Theorem:''' If ''R'' is a [[filtered algebra|filtered ring]] whose associated graded ring gr(''R'') is a domain, then ''R'' itself is a domain.
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| This theorem needs to be complemented by the analysis of the [[graded algebra|graded ring]] gr(''R'').
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| == Examples ==
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| * The ring ''nZ'' is a domain (for each integer ''n > 1'') but not an integral domain since <math>1 \not \in n\mathbb{Z}</math>.<ref name="Lanski" />
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| * The [[quaternions]] form a noncommutative domain. More generally, any [[division algebra]] is a domain, since all its non-zero elements are [[invertible]].
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| * The set of all [[integral quaternion]]s is a noncommutative ring which is a subring of quaternions, hence a noncommutative domain.
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| * The [[matrix ring]] of order greater than one is never a domain, since it has zero divisors, and even [[nilpotent]] elements. For example, the square of the [[matrix unit]] ''E''<sub>12</sub> is zero.
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| * The [[tensor algebra]] of a [[vector space]], or equivalently, the algebra of polynomials in noncommuting variables over a field, <math> \mathbb{K}\langle x_1,\ldots,x_n\rangle, </math> is a domain. This may be proved using an ordering on the noncommutative monomials.
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| * If ''R'' is a domain and ''S'' is an [[Ore extension]] of ''R'' then ''S'' is a domain.
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| * The [[Weyl algebra]] is a noncommutative domain. Indeed, it has two natural filtrations, by the degree of the derivative and by the total degree, and the associated graded ring for either one is isomorphic to the ring of polynomials in two variables. By the theorem above, the Weyl algebra is a domain.
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| * The [[universal enveloping algebra]] of any [[Lie algebra]] over a field is a domain. The proof uses the standard filtration on the universal enveloping algebra and the [[Poincaré–Birkhoff–Witt theorem]].
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| == Group rings and the zero divisor problem ==
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| Suppose that ''G'' is a [[group (mathematics)|group]] and ''K'' is a [[field (mathematics)|field]]. Is the [[group ring]] ''R'' = ''K''[''G''] a domain? The identity
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| : <math> (1-g)(1+g+\ldots+g^{n-1})=1-g^n,</math>
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| shows that an element ''g'' of finite [[order (group theory)|order]] ''n'' induces a zero divisor 1−''g'' in ''R''. The '''zero divisor problem''' asks whether this is the only obstruction, in other words,
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| : Given a [[field (mathematics)|field]] ''K'' and a [[torsion-free group]] ''G'', is it true that ''K''[''G''] contains no zero divisors?
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| No counterexamples are known, but the problem remains open in general (as of 2007).
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| For many special classes of groups, the answer is affirmative. Farkas and Snider proved in 1976 that if ''G'' is a torsion-free [[polycyclic group|polycyclic-by-finite]] group and char ''K'' = 0 then the group ring ''K''[''G''] is a domain. Later (1980) Cliff removed the restriction on the characteristic of the field. In 1988, Kropholler, Linnell and Moody generalized these results to the case of torsion-free [[solvable group|solvable]] and solvable-by-finite groups. Earlier (1965) work of [[Michel Lazard]], whose importance was not appreciated by the specialists in the field for about 20 years, had dealt with the case where ''K'' is the ring of [[p-adic integers]] and ''G'' is the ''p''th [[congruence subgroup]] of ''GL''(''n'','''Z''').
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| == See also ==
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| * [[Zero divisor]]
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| * [[Zero-product property]]
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| * [[Divisor (ring theory)]]
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| == Notes ==
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| {{reflist}}
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| == References ==
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| * {{Cite book | last1=Lam | first1=Tsit-Yuen | title=A First Course in Noncommutative Rings | publisher=[[Springer-Verlag]] | location=Berlin, New York | edition=2nd | isbn=978-0-387-95325-0 | mr=1838439 | year=2001}}
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| * {{cite book | author=Charles Lanski | title=Concepts in abstract algebra | publisher=AMS Bookstore | year=2005 | isbn=0-534-42323-X }}
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| * {{cite book | author=César Polcino Milies | author2=Sudarshan K. Sehgal | title=An introduction to group rings | publisher=Springer | year=2002 | isbn=1-4020-0238-6 }}
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| * {{cite book | author=Nathan Jacobson | title=Basic Algebra I | publisher=Dover | year=2009 | isbn=978-0-486-47189-1 }}
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| * {{cite book | author=Louis Halle Rowen | title=Algebra: groups, rings, and fields | publisher=[[A K Peters]] | year=1994 | isbn=1-56881-028-8 }}
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| {{DEFAULTSORT:Domain (Ring Theory)}}
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| [[Category:Ring theory]]
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| [[Category:Algebraic structures]]
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