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13-00-UnitiesOfRingAndSubring.tex
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13-00-UnitiesOfRingAndSubring.tex
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\documentclass[12pt]{article}
\usepackage{pmmeta}
\pmcanonicalname{UnitiesOfRingAndSubring}
\pmcreated{2013-03-22 14:49:37}
\pmmodified{2013-03-22 14:49:37}
\pmowner{pahio}{2872}
\pmmodifier{pahio}{2872}
\pmtitle{unities of ring and subring}
\pmrecord{5}{36491}
\pmprivacy{1}
\pmauthor{pahio}{2872}
\pmtype{Result}
\pmcomment{trigger rebuild}
\pmclassification{msc}{13-00}
\pmclassification{msc}{16-00}
\pmclassification{msc}{20-00}
\pmrelated{UnityOfSubring}
\endmetadata
% this is the default PlanetMath preamble. as your knowledge
% of TeX increases, you will probably want to edit this, but
% it should be fine as is for beginners.
% almost certainly you want these
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
% used for TeXing text within eps files
%\usepackage{psfrag}
% need this for including graphics (\includegraphics)
%\usepackage{graphicx}
% for neatly defining theorems and propositions
%\usepackage{amsthm}
% making logically defined graphics
%%%\usepackage{xypic}
% there are many more packages, add them here as you need them
% define commands here
\begin{document}
Let $R$ be a ring and $S$ a proper subring of it. \,Then there exists five cases in all concerning the possible unities of $R$ and $S$.
\begin{enumerate}
\item $R$ and $S$ have a common unity.
\item $R$ has a unity but $S$ does not.
\item $R$ and $S$ both have their own non-zero unities but these are distinct.
\item $R$ has no unity but $S$ has a non-zero unity.
\item Neither $R$ nor $S$ have unity.
\end{enumerate}
\textbf{Note:} \, In the cases 3 and 4, the unity of the subring $S$ must be a zero divisor of $R$.
\textbf{Examples}
\begin{enumerate}
\item The ring $\mathbb{Q}$ and its subring $\mathbb{Z}$ have the common
unity 1.
\item The subring $S$ of even integers of the ring $\mathbb{Z}$ has no unity.
\item Let $S$ be the subring of all pairs $(a,\,0)$ of the ring
\,$R = \mathbb{Z}\times\mathbb{Z}$\, for which the operations ``$+$'' and ``$\cdot$'' are defined componentwise (i.e. \,$(a,\,b)+(c,\,d) = (a+c,\,b+d)$\, etc.). \,Then $S$ and $R$ have the unities $(1,\,0)$ and $(1,\,1)$, respectively.
\item Let $S$ be the subring of all pairs $(a,\,0)$ of the ring \,$R = \{(a,\,2b)|\,\,\,a\in\mathbb{Z}\,\land \,b\in\mathbb{Z}\}$ (operations componentwise). \,Now $S$ has the unity $(1,\,0)$ but $R$ has no unity.
\item Neither the ring $\{(2a,\,2b)|\,\,\,a,\,b\in\mathbb{Z}\}$ (operations componentwise) nor its subring consisting of the pairs $(2a,\,0)$ have unity.
\end{enumerate}
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\end{document}