Continuing How to add or remove space in math mode with 50% stretching and shrinking? and feeding
\documentclass{article}
\pagestyle{empty}
%% Math skips with glue being half of the modulus of the value of the argument (cf. http://tex.stackexchange.com/a/669865):
\newcommand{\flexibleMSkip}[1]{%%% 50 per cent of the modulus of the argument after plus and minus. The argument may be any integer or floating-point number.
\mskip#1
plus \muexpr(\ifdim\mutoglue\muexpr#1<0pt -\fi#1)/2\relax
minus \muexpr(\ifdim\mutoglue\muexpr#1<0pt -\fi#1)/2\relax
\relax
}
\begin{document}\noindent
\(V,\)\\
\(V\flexibleMSkip{-.5mu},\)\\
\(V\mskip-.5mu,\)\\
\(V\mkern-.5mu,\)\
\end{document}
to pdflatex yields
The result is counterintuitive: we wish to have tighter spacing and, therefore, add negative space (as glue or kern), but we get a wider output instead.
Probably, the standard kern between V and , is forgotten as soon as we add our own, explicit glue or kern. Any way to retain the standard kern and then add our own glue? In other words, we wish to have a macro, say, \flexibleMSkipRetainingOriginalKern such that the spacing between the two symbols produced by \flexibleMSkipRetainingOriginalKern{-.5mu} is -0.5mu plus.25mu minus.25mu larger (in particular, if there's no need for stretching or shrinking, simply 0.5mu smaller) than the spacing between two the symbols produced by , for all symbols and .