My symbols are rendered as ff. Have I ran out of memory?

Question

I have been trying to solve a 4x4 matrix and have done a hell a lot of writing. Now I am near the end and I am not getting the requested output out of the LaTeX compiler.

When I write for example:

\begin{minipage}[t]{\textwidth}
\begin{equation}\label{e50}
R^{-1} = \frac{1}{\left|R\right|} \rm{adj}(R) = \alpha
\end{equation}
\end{minipage}

the last symbol isn't shown as "alpha" but rather as a very weird symbol ff like I show in the picture below:

I get the same weird symbol for any symbol that I put in place of \alpha or even after it. From this point on all of my symbols are typeset as ff.

If I put same source code in my other new document all of my symbols again render as they were supposed to.

Q1: Have I ran out of memory?

Q2: How can I fix this?

Answer 1

You shouldn't be getting ff instead of \alpha. But your input has a bad error anyway: \rm is not a command with an argument, but rather a declaration that extends its influence until the group in which it's been issued ends, in this case up to \end{equation}.

The command \rm must not be used. Forget it, together with \it, \bf and \tt. They are obsolete and, as in your case, can be misunderstood by users.

In place of them use \textrm, \textit, \texttt and the others that you find in any user guide. In math there are \mathrm and some similar commands, but for operator names there's a better way: say

\usepackage{amsmath}
\DeclareMathOperator{\adj}{adj}

in your preamble and then your formula can be input as

\begin{equation}\label{e50}
R^{-1} = \frac{1}{\lvert R\rvert} \adj(R) = \alpha
\end{equation}

The minipage you use is not needed and probably makes vertical spacing awkward. Note also \lvert and \rvert instead of \left| and `\right|.