5

I'd like to do the same thing that is been done to the greek letter \chi in this post to the letter g. In particular I'd like to redefine g in mathmode to work as \chi here:

   \makeatletter
   \renewcommand\chi{\@ifnextchar_\sub@chi\latexchi}
   \newcommand{\sub@chi}[2]{% #1 is _, #2 is the subscript
     \@ifnextchar^{\subsup@chi{#2}}{\latexchi^{}_{#2}}%
   }
   \newcommand{\subsup@chi}[3]{% #1 is the subscript, #2 is ^, #3 is the superscript
     \latexchi_{#1}^{#3}%
   }
   \makeatother
6

You can do better than six years ago.

I define a generic macro for lowering the subscript as if a superscript is present.

For g, one needs to make it math active and supply a definition for it, with the common \lowercase trick. In order to save the standard math g, we need to copy its math code.

\documentclass{article}
\usepackage{amsmath,xparse}

\linespread{1.1} % high subscripts and low subscripts

\NewDocumentCommand{\movedownsub}{e{^_}}{%
  \IfNoValueTF{#1}{%
    \IfNoValueF{#2}{^{}}% neither ^ nor _, do nothing; if no ^ but _, add ^{}
  }{%
    ^{#1}% add superscript if present
  }%
  \IfNoValueF{#2}{_{#2}}% add subscript if present
}
% chi
\NewCommandCopy{\latexchi}{\chi}
\RenewDocumentCommand{\chi}{}{\latexchi\movedownsub}
% g
\mathchardef\latexmathg=\mathcode`g
\begingroup\lccode`~=`g \lowercase{\endgroup\def~}{\latexmathg\movedownsub}
\mathcode`g="8000

\begin{document}

\textbf{Chi}

$\latexchi_{A}\chi_{A}$

$\latexchi_{A_{n}}\chi_{A_{n}}$

$\chi_{A}\quad\chi_{A}^{2}\quad\chi^{2}_{A}\quad\chi^{2}\quad\chi$

\medskip

\textbf{g}

$\latexmathg_{A}g_{A}$

$\latexmathg_{A_{n}}g_{A_{n}}$

$g_{A}\quad g_{A}^{2}\quad g^{2}_{A}\quad g^{2}\quad g$

\end{document}

enter image description here

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