@ symbol for index keys, when using imakeidx package

The @ character is indeed reserved for MakeIndex processing; it is used for entries such as

\index{abc@\textit{abc}} 

where the text to be indexed has formatting and we want to specify a key for sorting purposes.

You have two possibilities. The first is quoting the @ by typing

\index{"@variable} 

(but this may fail if you load babel and a language that makes " into a shorthand prefix).

The second strategy is adding

\begin{filecontents*}{\jobname.mst} actual '=' \end{filecontents*} 

at the very beginning of your main TeX file (before \documentclass is the best place).

This assumes = is never used in your entries. The “sort key” entries like the example above can be input using = instead of @

\index{abc=\textit{abc}} 

and \index{@variable} will now be sorted among the symbols.

The trick is that the filecontents* environment will write a file named <jobname>.mst (where <jobname> is the main TeX file name) and MakeIndex looks for it when processing a file.

However, this will prevent loading an .ist files, so beware. If you pass no option to MakeIndex, the method will work.