%%% Bogus\l{}aw Jackowski & Piotr Strzelczyk: sigma tweak
% A sum operator that adjusts its width automatically to the widest
% subscript or superscript -- perhaps completely useless ;-)
% Note: pdfTeX is required for clippling; you may wish to use
%          other tools for this purpose

% defining the parts of the symbol
\def\SIGMAleft{\copy0 }
\def\SIGMAright{\copy1 }
\def\SIGMAcenter{\copy2 }

% preparing the parts of the symbol
\setbox0\hbox{\tenex \char88\kern-.6em}
\setbox1\hbox{\kern-.8em\tenex \char88}
\setbox2\hbox{\hbox{\kern-.8em\tenex \char88\kern-.6em}}

% clipping:
\pdfxform0 \setbox0\hbox{\pdfrefxform\pdflastxform}
\pdfxform1 \setbox1\hbox{\pdfrefxform\pdflastxform}
\pdfxform2 \setbox2\hbox{\pdfrefxform\pdflastxform}

% assembling:
\def\SIGMA{%
  \mathop{\hbox{\SIGMAleft \kern-.1em
  \xleaders\SIGMAcenter\hfill
  \kern-.1em\SIGMAright}}\limits}% with \nolimits behaves ``traditionally''

$ 
  \SIGMA^{\rm a}\quad
  \SIGMA^{\rm sum}\quad
  \SIGMA^{\rm operator}\quad
  \SIGMA^{\rm that\ adjusts}\quad
  \SIGMA^{\rm its\ width\ automatically}
        _{\rm to\ the\ widest\ subscript\ or\ superscript} 
$

\end