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commutative_rings [2010/07/29 15:46] (current)
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 +=====Commutative rings=====
 +Abbreviation: **CRng**
 +====Definition====
 +A \emph{commutative ring} is a [[rings]] $\mathbf{R}=\langle R,+,-,0,\cdot\rangle$ such that
 +
 +$\cdot$ is commutative:  $x\cdot y=y \cdot x$
 +
 +
 +Remark:  $Idl(R)=\{ all ideals of R\}$
 +
 +$I$ is an ideal if $a,b\in I\Longrightarrow a+b\in I$
 + 
 +and $\forall r \in R\ (r\cdot I\subseteq I)$
 +
 +
 +==Morphisms==
 +Let $\mathbf{R}$ and $\mathbf{S}$ be commutative rings with identity. A morphism from $\mathbf{R}$
 +to $\mathbf{S}$ is a function $h:R\rightarrow S$ that is a homomorphism:
 +
 +$h(x+y)=h(x)+h(y)$, $h(x\cdot y)=h(x)\cdot h(y)$
 +
 +Remark:
 +It follows that $h(0)=0$ and $h(-x)=-h(x)$.
 +
 +====Examples====
 +Example 1: $\langle\mathbb{Z},+,-,0,\cdot\rangle$, the ring of integers with addition, subtraction, zero, and multiplication.
 +
 +
 +====Basic results====
 +$0$ is a zero for $\cdot$: $0\cdot x=x$ and $x\cdot 0=0$.
 +
 +====Properties====
 +^[[Classtype]]  |variety |
 +^[[Equational theory]]  |decidable |
 +^[[Quasiequational theory]]  | |
 +^[[First-order theory]]  |undecidable |
 +^[[Locally finite]]  |no |
 +^[[Residual size]]  |unbounded |
 +^[[Congruence distributive]]  |no |
 +^[[Congruence modular]]  |yes |
 +^[[Congruence n-permutable]]  |yes, $n=2$ |
 +^[[Congruence regular]]  |yes |
 +^[[Congruence uniform]]  |yes |
 +^[[Congruence extension property]]  | |
 +^[[Definable principal congruences]]  | |
 +^[[Equationally def. pr. cong.]]  | |
 +^[[Amalgamation property]]  | |
 +^[[Strong amalgamation property]]  | |
 +^[[Epimorphisms are surjective]]  | |
 +====Finite members====
 +
 +$\begin{array}{lr}
 +f(1)= &1\\
 +f(2)= &2\\
 +f(3)= &2\\
 +f(4)= &9\\
 +f(5)= &2\\
 +f(6)= &4\\
 +[http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=A037289 Finite commutative rings in the Encyclopedia of Integer Sequences]
 +\end{array}$
 +
 +====Subclasses====
 +[[Commutative rings with identity]]
 +
 +[[Fields]]
 +
 +====Superclasses====
 +[[Rings]]
 +
 +
 +====References====
 +
 +[(Ln19xx>
 +)]