retractRetract2002-02-02 23:00:192004-11-11 15:06:59Definitionretraction% 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 hereLet $X$ be a topological space and $Y$ a subspace of $X$. If there exists a continuous map $r:X \to Y$ such that $r(y)=y$ for all $y \in Y$, then we say $Y$ is a \emph{retract} of $X$ and $r$ is a \emph{retraction}.