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<P ALIGN=RIGHT><FONT SIZE=1>Xy-pic User's Guide</FONT> <A HREF="18pt.html"><IMG SRC="./quest.gif" ALIGN=TOP HEIGHT=27 WIDTH=21 BORDER=0 ALT="?"></A><A NAME="basicslabels" NAME="tex2html140" HREF="node6.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="previous" SRC="./previous.gif"></A><A NAME="tex2html144" HREF="node6.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="previous" SRC="./previoussection.gif"></A><A NAME="tex2html146" HREF="node3.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="up" SRC="./up.gif"></A><A NAME="tex2html142" HREF="node8.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="next" SRC="./nextsection.gif"></A><A NAME="tex2html148" HREF="node8.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="next" SRC="./next.gif"></A></P><H2><A NAME="SECTION00034000000000000000">1.4 Labels</A></H2> You can put labels on arrows. Labels are conceptualized as
sub- and superscripts on arrows such that they are placed in the
usual positions (as "limits"), <I>i.e.</I>, <code>^</code> reads "above" and
<code>_</code> "below" on an arrow pointing right. Notice that the
positions depend <em>only</em> on the direction of the arrow, the
absolute notions of "up," "down," etc. are not important. For
example,
<BLOCKQUOTE><PRE>$\xymatrix@1{
X\ar[r]^a_b & Y & Z\ar[l]^A_B }$</PRE></BLOCKQUOTE>
will set <IMG WIDTH=152 HEIGHT=32 ALIGN=BOTTOM ALT="tex2html_wrap2488" SRC="img6.gif"> (the <code>@1</code> is a special code that can be
used for "one-line" diagrams to improve the placement on the line;
more such spacing codes are described in section <A HREF="node24.html#entriesresizing">3.3</A>).
<P>
It is possible to use labels that are not single letters, digits, or
control sequences: if a simple math formula in the default style
(script style) is desired then simply enclose in <code>{</code>...<code>}</code>. In
practice anything can be used as a label as described in
section <A HREF="node13.html#arrowsobjects">2.2</A>.
<P>
Each label is placed perpendicular to the arrow at the point halfway
between the centers of the base and target objects. This is usually
the most �hethic, however, in diagrams where the sizes of the
entries vary much it is sometimes nicer to place the label at the
center of the actual arrow. This
behaviour is requested by inserting a <code>-</code> (minus) right after the
<code>^</code> or <code>_</code>:
<IMG WIDTH=204 HEIGHT=22 ALIGN=BOTTOM ALT="tex2html_wrap2490" SRC="img7.gif"> was typeset by
<BLOCKQUOTE><PRE>$\xymatrix@1{
A\times B\times C\times D \ar[r]^-{+} &B
}$</PRE></BLOCKQUOTE>
(it becomes
<IMG WIDTH=204 HEIGHT=22 ALIGN=BOTTOM ALT="tex2html_wrap2492" SRC="img8.gif">without the <code>-</code>). In fact <code>-</code> is in just one of the may
possible placings of labels described in section <A HREF="node12.html#arrowsplaces">2.1</A>.
<P>
<A NAME="Exercise3"> </A><H4><A NAME="SECTION00034010000000000000">Exercise 3.</A></H4> Typeset the second axiom of category theory as
<P><CENTER><BR><IMG WIDTH=233 HEIGHT=108 ALIGN=BOTTOM ALT="displaymath176" SRC="img9.gif"><BR></CENTER><P>
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<A HREF="node33.html#Answer3">Answer.</A>
<P>
<P> <P ALIGN=RIGHT><FONT SIZE=1>Xy-pic User's Guide</FONT> <A HREF="18pt.html"><IMG SRC="./quest.gif" ALIGN=TOP HEIGHT=27 WIDTH=21 BORDER=0 ALT="?"></A><A NAME="tex2html140" HREF="node6.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="previous" SRC="./previous.gif"></A><A NAME="tex2html144" HREF="node6.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="previous" SRC="./previoussection.gif"></A><A NAME="tex2html146" HREF="node3.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="up" SRC="./up.gif"></A><A NAME="tex2html142" HREF="node8.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="next" SRC="./nextsection.gif"></A><A NAME="tex2html148" HREF="node8.html"><IMG WIDTH=27 HEIGHT=27 BORDER=0 ALIGN=ABSMIDDLE ALT="next" SRC="./next.gif"></A><P><ADDRESS>
<BR><HR>
<a href="COPYING/index.html">�/a> <a href="http://www.brics.dk/~krisrose/">Kristoffer H. Rose</a>
<<a href="mailto:krisrose@brics.dk">krisrose@brics.dk</a>>
January 6, 1997
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