making new symbol from two symbols
I would use \mkern
instead of \kern
, because it uses unit mu
, which follows the math style:
\documentclass{article}
\newcommand*{\doublenabla}{%
{\nabla\mkern-12mu\nabla}% subformula acts as \mathord
}
\begin{document}
\[ \doublenabla_{\doublenabla_{\doublenabla}} \]
\end{document}
Adjust the shift value to your needs.
Another possibility using mathtools
to specify the offset from the left rather than from the right produces slightly different spacing:
\documentclass{article}
\usepackage{mathtools}
\newcommand{\nnabla}{\mathrlap{\nabla}\,\nabla}
\begin{document}
$\nnabla_{\nnabla_{\nnabla}}$
\end{document}