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[parent] ring adjunction (Definition)

Let $ R$ be a commutative ring and $ E$ an extension ring. If $ \alpha \in E$ and commutes with all elements of $ R$, then the smallest subring of $ E$ containing $ R$ and $ \alpha$ is denoted by $ R[\alpha]$. We say that $ R[\alpha]$ is obtained from $ R$ by adjoining $ \alpha$ to $ R$ via ring adjunction.

By the Theorem 1 about “evaluation homomorphism”,

$\displaystyle R[\alpha] = \{f(\alpha)\mid \, f(X)\in R[X]\},$
where $ R[X]$ is the polynomial ring in one indeterminate over $ R$. Therefore, $ R[\alpha]$ consists of all expressions which can be formed of $ \alpha$ and elements of the ring $ R$ by using additions, subtractions and multiplications.

Examples: The polynomial rings $ R[X]$, the ring $ \mathbb{Z}[i]$ of the Gaussian integers, the ring $ \mathbb{Z}[\frac{-1+i\sqrt{3}}{2}]$ of Eisenstein integers.



"ring adjunction" is owned by pahio.
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See Also: generated subring, finite ring has no proper overrings, ground fields and rings, polynomial ring over integral domain, a condition of algebraic extension


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field adjunction (Definition) by pahio
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Cross-references: Eisenstein integers, Gaussian integers, multiplications, subtractions, additions, expressions, indeterminate, polynomial ring, subring, ring, extension, commutative ring
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This is version 13 of ring adjunction, born on 2004-05-27, modified 2008-03-08.
Object id is 5874, canonical name is RingAdjunction.
Accessed 1697 times total.

Classification:
AMS MSC13B02 (Commutative rings and algebras :: Ring extensions and related topics :: Extension theory)
 13B25 (Commutative rings and algebras :: Ring extensions and related topics :: Polynomials over commutative rings)

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