tex dot
で検索して、
\documentclass{jarticle}

\makeatletter
\def\iddots{\mathinner{\mkern1mu\raise\p@
\hbox{.}\mkern2mu\raise4\p@\hbox{.}\mkern2mu
\raise7\p@\vbox{\kern7\p@\hbox{.}}\mkern1mu}}
\def\adots{\mathinner{\mkern2mu\raise\p@\hbox{.} %% yhmath.styから
\mkern2mu\raise4\p@\hbox{.}\mkern1mu
\raise7\p@\vbox{\kern7\p@\hbox{.}}\mkern1mu}}
\makeatother

\begin{document}


$$
x^{x^{x^{…^{x}}}}
x^{x^{x^{\iddots^{x}}}}
x^{x^{x^{\adots^{x}}}}
$$


\end{document}