3n+1

collatz.tex

\documentclass{article}
\newcount\x
\newcount\i
\def\collatz#1{
	\x #1 \relax
	\calccollatz
}
\def\calccollatz{
	\ifnum\x=1
		\the\x
	\else
		\the\x,
		\ifodd\x
			\multiply\x by 3
			\advance\x by 1
		\else
			\divide\x by 2
		\fi
		\calccollatz
	\fi
}
\begin{document}
	\i = 1
	\loop\ifnum\i<11
		\noindent \collatz\i \par
		\advance\i by 1
	\repeat
\end{document}