Run the following through TeX and it will page-number with primes.
\pageno=2
\newcount\p \newcount\q \newcount\flag
\def\advancepageno{
\loop
\global\advance\pageno by 1 \p=1
{\loop
\advance\p by 1
\q=\pageno \divide\q by \p \multiply\q by \p
\ifnum\q=\pageno
\global\flag=0
\else
\global\flag=1
\q=\p \multiply\q by \p
\fi
\ifnum\q<\pageno \repeat}
\ifnum\flag=0\repeat}
% Now some dummy content.
\loop
\centerline{This is page \the\pageno} \vfil\eject
\ifnum\pageno<100\repeat
\end
This is not a good program, algorithmically or stylistically, it is just a very simple example of TeX as a programming language.