Trying to use multiple vertices with math mode names

Question

I am working on a simple graph, using TikZ.

The code I have so far is:

\documentclass[10pt]{article}
\usepackage{tikz}
\usetikzlibrary{graphs,graphs.standard}
\begin{document}
\begin{center}
\begin{tikzpicture}
\graph[clockwise, n=3, V={i,j,k}, edge={bend left=45, -stealth}] { subgraph C_n };
\end{tikzpicture}
\end{center}
\end{document}

The output I receive while compiling the code above is:

The only change I want to make is to display $\mathbf{i}$ instead of i, $\mathbf{j}$ instead of j and $\mathbf{k}$ instead of k. When I try to do so, compiling doesn't work and I receive the following error message:

Undefined control sequence.

Answer 1

The list of names you specify for the key V is used for the names of the nodes. These kind of names don't allow “special” macros like \mathbf. (Node names are used as macro names internally by PGF and these need to be fully expandable to text. \mathbf is not and all hell breaks loose because of that.)

However, the graphs library provides the typeset key where you have access to certain macros, mainly the \tikzgraphnodename and the \tikzgraphnodetext macros (which are the same in this simple case):

\graph[
  clockwise,
  n=3,
  V={i,j,k},
  edge={bend left=45, -stealth},
  typeset=$\mathbf{\tikzgraphnodetext}$ % !or $\mathbf{\tikzgraphnodename}$
] { subgraph C_n };

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}
\begin{document}
\tikz\graph[clockwise, n=3, V={i,j,k}, edge={bend left=45, -stealth}, % no typeset
  ] { subgraph C_n };
\tikz\graph[clockwise, n=3, V={i,j,k}, edge={bend left=45, -stealth}, % typeset with text
  typeset=$\mathbf{\tikzgraphnodetext}$] { subgraph C_n };
\tikz\graph[clockwise, n=3, V={i,j,k}, edge={bend left=45, -stealth}, % typeset with name
  typeset=$\mathbf{\tikzgraphnodename}$] { subgraph C_n };
\tikz\graph[clockwise, n=3, edge={bend left=45, -stealth}, % for fun
  typeset=$v_{\tikzgraphnodetext}$,
  nodes={text=red!\the\numexpr\tikzgraphnodetext00/3\relax!blue}
] { subgraph C_n };
\end{document}

---

Thanks to the comment of Henri Menke, a small fix to the subgraph I_n (which is used internally by subgraph C_n) enables you to use the full power of the \graph way of specifying a vertex.

However, as it uses a \foreach loop you need to protect , when they're used the node text or in the options (or in the node name but that's not a good idea to begin with).

V={ i / $\mathbf{i}$, j / $\mathbf{j}$, k / $\mathbf{k}$ [{red, fill=green}]}

or

V={ i / $\mathbf{i}$, j / $\mathbf{j}$, {k / $\mathbf{k}$ [red, fill=green]}}

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{graphs,graphs.standard}
\makeatletter
\tikzgraphsset{
  declare={subgraph I_n}{
    \foreach \tikz@lib@graph@node@num in \tikzgraphV
      {[parse/.expand once=\tikz@lib@graph@node@num]}}}
\makeatother
\begin{document}
\tikz\graph[clockwise, n=3,
  V={ i / $\mathbf{i}$, j / $\mathbf{j}$, k / $\mathbf{k}$ },
  edge={bend left=45, -stealth}] { subgraph C_n };
\end{document}