> why can't I combine the two expressions like this?
> \[
> (L_i < L_j < R_i) ~\arl \arr~ \hbox{i is the ancestor of j}
>
> (L_i < R_j < R_i) ~\arl \arr~ \hbox{i is the ancestor of j}
> \]

\[ \] is wrong. Check your manuals again. It sounds like you want {eqnarray*}. Really, nobody wants to be used to regurgitate basic instructions that are given in any manual.

Secondly (and the reason I bothered replying) is that the \hbox answer is seriously deficient. It should be \mbox{$i$ is the ancestor of $j$} You could also write: i\mbox{ is the ancestor of }j but that is awful. If you are using amsmath (amstext) then even better is \text{$i$ is the ancestor of $j$}

Thirdly, \arl\arr should probably be \rightleftarrow. Do you mean "implies"? That is usually \Rightleftarrow.

Donald Arseneau

\[ .. \] is intended for single equations; if you want more than one equation in a single displaymath environment, you should use the eqnarray env:

\begin{eqnarray*} %the * is to prevent numbering (L_i < L_j < R_i) ~\arl \arr~ \hbox{i is the ancestor of j}

(L_i < R_j < R_i) ~\arl \arr~ \hbox{i is the ancestor of j} \end{eqnarray*}

And by the way, instead of ~\arl \arr~ you should use \iff

(which is \Leftrightarrow plus some spacing; if you really want single implication arrows---not common nowadays--- you could grab the definiton of \iff and replay it with \leftrightarrow instead of \Leftrightarrow)

Giuseppe Bilotta

\begin{eqnarray*} %the * is to prevent numbering
ac & = & bd \\
c & = & d \\   % '=' sign aligned
a  =  b \\     % '=' sign not aligned
\end{eqnarray*}