\def\IR{\hbox{$I\mkern-6muR$}}
\def\IP{\hbox{$I\mkern-6muP$}}
\def\IN{\hbox{$I\mkern-6muN$}}
\font\sqi=cmssqi8
\def\IC{\hbox{$C\mkern-13.5mu
\raise0.49pt\hbox{\sqi I}\mkern9mu$}}