Sstrhsrtj

Why does the widehat mathematical symbol show up as follows in the MWE below?

documentclass{scrartcl}

usepackage{bm}

usepackage{amsmath}

newcommand*{xv}{operatorname{xv}}

usepackage[noautomatic]{imakeidx}

makeindex[intoc]



newcommand*lettergroupDefault[1]{textbf{sffamily Symbols}}

newcommand*lettergroup[1]{%

  partextbf{sffamily#1}par

  nopagebreak

}



begin{document}

begin{align*}

  E=mc^2index{. xv@$widehat{xv}_n$}

end{align*}

printindex

end{document}

If you look in the .idx file, you will find

indexentry{. xv@$mathaccent "0362relax {xv  }_n$}{1}

which fails because " is a quoting character in .idx files, so this is interpreted as if it were mathaccent 362 changing hexadecimal into decimal.

A way out could be protectwidehat, but I'd not recommend this. The problem is that

widehat{xv}

does not behave as an operator as far as spacing is concerned, because it ends up being an Acc atom, which is later treated as an ordinary one. Try with

begin{align*}

  xv A &= 2 \

  hxv A &= 2 \

  widehat{xv} A &= 2

end{align*}

where xv and hxv are properly defined by

DeclareMathOperator{xv}{xv}

DeclareMathOperator{hxv}{widehat{xv}}

The output is

and you can immediately see the problem.

Using hxv in the index command will write hxv (and similarly xv will write itself) instead of doing nasty expansions. The proposed definition of xv is to be preferred to newcommand{xv}{operatorname{xv}} also for this reason.

Beware that the same problem with " will appear with delimiters: protect also things such as lbrace.