I've defined the following new commands:
\newcommand{\atoi}{\mathrm{\rotatebox[origin=c]{180}{$\iotaup$}}} \newcommand{\scriptatoi}{{\scriptsize{\mathrm{\rotatebox[origin=c]{180}{$\iotaup$}}}}} to use in this formula (and other formulas in which $\atoi$ occurs):
$\psi^{\scriptatoi x\phi}_{z} \equiv \exists y(\phi^y_x \:\&\: \forall u(\phi^u_x \rightarrow u\!=\!y) \:\&\: \psi^y_z)$ When I typeset the LaTeX source file using pdflatex (from TeXLive 2010), I get the following complaint:
LaTeX Font Warning: Command \scriptsize invalid in math mode on input line 352.
If, however, I write the following line instead:
$\psi^{\atoi x\phi}_{z} \equiv \exists y(\phi^y_x \:\&\: \forall u(\phi^u_x \rightarrow u\!=\!y) \:\&\: \psi^y_z)$ the atoi character appears full size, not superscript size.
Writing
$\psi^{\scriptsize\atoi x\phi}_{z} \equiv \exists y(\phi^y_x \:\&\: \forall u(\phi^u_x \rightarrow u\!=\!y) \:\&\: \psi^y_z)$ again yields the desired typeset result, and again yields the LaTeX Font Warning.
How can I do what I want, that is, get my new symbol in an appropriate superscript size when using it in a superscript, while respecting the font restrictions of math mode?
\scriptstyle. Generally,\...sizeapplies to textmode, whereas\...styleapplies to mathmode.