Take any natural number m > 0.
n:=m;
repeat
if (n is odd) then n:=3*n+1; else n:=n/2;
until (n==1)
The conjecture has been verified for all numbers up to 
.
References
Unsolved Problems in Number Theory. Richard K Guy. Springer, Problem E16.
Elementary Number Theory. Underwood Dudley. 2nd ed.
G.T. Leavens and M. Vermeulen 3x+1 search programs ] Comput. Math. Appl.
vol. 24 n. 11 (1992), 79-99.