Principal ideal

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In mathematics, specifically ring theory, a principal ideal is an ideal in a ring that is generated by a single element of through multiplication by every element of The term also has another, similar meaning in order theory, where it refers to an (order) ideal in a poset generated by a single element which is to say the set of all elements less than or equal to in

Contents

The remainder of this article addresses the ring-theoretic concept.

Definitions

While the definition for two-sided principal ideal may seem more complicated than for the one-sided principal ideals, it is necessary to ensure that the ideal remains closed under addition. [1] :251–252

If is a commutative ring, then the above three notions are all the same. In that case, it is common to write the ideal generated by as or

Examples and non-examples

A ring in which every ideal is principal is called principal, or a principal ideal ring . A principal ideal domain (PID) is an integral domain in which every ideal is principal. Any PID is a unique factorization domain; the normal proof of unique factorization in the integers (the so-called fundamental theorem of arithmetic) holds in any PID.

As an example, is a principal ideal domain, which can be shown as follows. Suppose where and consider the surjective homomorphisms Since is finite, for sufficiently large we have Thus which implies is always finitely generated. Since the ideal generated by any integers and is exactly by induction on the number of generators it follows that is principal.

Properties

Any Euclidean domain is a PID; the algorithm used to calculate greatest common divisors may be used to find a generator of any ideal. More generally, any two principal ideals in a commutative ring have a greatest common divisor in the sense of ideal multiplication. In principal ideal domains, this allows us to calculate greatest common divisors of elements of the ring, up to multiplication by a unit; we define to be any generator of the ideal

For a Dedekind domain we may also ask, given a non-principal ideal of whether there is some extension of such that the ideal of generated by is principal (said more loosely, becomes principal in ). This question arose in connection with the study of rings of algebraic integers (which are examples of Dedekind domains) in number theory, and led to the development of class field theory by Teiji Takagi, Emil Artin, David Hilbert, and many others.

The principal ideal theorem of class field theory states that every integer ring (i.e. the ring of integers of some number field) is contained in a larger integer ring which has the property that every ideal of becomes a principal ideal of In this theorem we may take to be the ring of integers of the Hilbert class field of ; that is, the maximal unramified abelian extension (that is, Galois extension whose Galois group is abelian) of the fraction field of and this is uniquely determined by

Krull's principal ideal theorem states that if is a Noetherian ring and is a principal, proper ideal of then has height at most one.

See also

Related Research Articles

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<span class="mw-page-title-main">Prime ideal</span> Ideal in a ring which has properties similar to prime elements

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References

  1. Dummit, David S.; Foote, Richard M. (2003-07-14). Abstract Algebra (3rd ed.). New York: John Wiley & Sons. ISBN   0-471-43334-9.