create or replace type int_tab_typ is table of integer;
create or replace function collatz(p_n in integer)
RETURN int_tab_typ PIPELINED as
if p_n < 1 or mod(p_n,1)>0 then
while n > 1
pipe row (n);
if mod(n,2)=1 then
n:=n / 2;
select * from table(collatz(101));
More on Collatz Conjecture (Wikipedia).
The point? Just for fun. Showed it to my 11 years old kid and asked him to figure out the 17 steps sequence of collatz(7) in his head. "Is this going to stop?", he asked half through. "That is what we are going to find out, nobody really knows", I told him.
If you have absolutely nothing useful to do, but have CPU and storage to waste, you may create a table containing n and the number of steps for collatz(n), with values of n ranging from 1 to 231-1 or until storage is exhausted, whichever comes first. Before doing so you may consider changing the function above to just count the steps and look up previous calculations from the table.