Adding a symbol to the top of a vertex in a connected tree

Question

I have a code for a tree

\[
\xymatrix@-1.25pc{
   *={\bullet}  &  &*={\bullet} &  
   \frm{}\\
   &*={\bullet}\ar@{-}[lu] \ar@{-}[ru]& & 
 }
\]

which will produce this

as its output. However, I would like to number the top nodes as follows:

yet I would like the line segments to be connected to the center of the vertices. I tried replacing \bullet with \stackrel{i}{\bullet} as follows:

\[
\xymatrix@-1.25pc{
*={\stackrel{1}{\bullet}}  &  &*={\stackrel{2}{\bullet}} &  
\frm{}\\
&*={\bullet}\ar@{-}[lu] \ar@{-}[ru]& & 
 }
\]

but the nodes are no longer connected to the edges:

which is not what I want.

score 4 · Accepted Answer · answered Sep 22 '19 at 22:26

4

A solution with TiKz. (I never used the xy package).

\documentclass[margin=3mm]{standalone}
\usepackage{tikz}
\tikzset{%
pics/bullet/.style args={#1,#2}{
    code={
    \node at (0,1) [draw,circle,fill,minimum size=2mm,inner sep=0pt,label={$#1$}](#1){};
    \node at (2,1) [draw,circle,fill,minimum size=2mm,inner sep=0pt,,label={$#2$}](#2){};
    \node at (1,0) [draw,circle,fill,minimum size=2mm,inner sep=0pt](bb){};
    \draw (bb)--(#1) (bb)--(#2);
    }}}
\begin{document}

\begin{tikzpicture}
\pic at (0,0) {bullet={1,2}};
\pic at (3,0) {bullet={3,4}};
\end{tikzpicture}

\end{document}

answered Sep 22 '19 at 22:26

2

I have leaved the question because I have thinked that the best solution was TikZ. Very good. – Sebastiano Sep 22 '19 at 22:45
2

@Sebastiano, thank you. – Sep 22 '19 at 23:12
1

:o) it is necessary, for me, many time to create with xy package to give an answer. But with xy package it is possible to draw two trees. – Sebastiano Sep 22 '19 at 23:16

score 4 · Answer 2 · answered Sep 23 '19 at 03:51

The XY syntax is quite concise and powerful, but it takes some getting used to. But it's easy to add the labels relative to the position of the bullet using the \save and \restore function of XY. See my answer to Pushout commutative diagram for a detailed explanation of how that works.

\documentclass{article}
\usepackage[all]{xy}
\begin{document}

\[
\xymatrix@-1.25pc{
   *={\bullet}\save+<0ex,2ex>*={1}\restore  &  &*={\bullet}\save+<0ex,2ex>*={2}\restore&  
   \frm{}\\
   &*={\bullet}\ar@{-}[lu] \ar@{-}[ru]& & 
 }
\]
\end{document}

Adding a symbol to the top of a vertex in a connected tree

2 Answers2