Just in case you ever need it: \ddots going the other direction:
\makeatletter
\def\Ddots{\mathinner{\mkern1mu\raise\p@
\vbox{\kern7\p@\hbox{.}}\mkern2mu
\raise4\p@\hbox{.}\mkern2mu\raise7\p@\hbox{.}\mkern1mu}}
\makeatother
Then you can say: \varepsilon_0= \omega^{\omega^{\Ddots}}
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