On the nonexistence of cycles for the Collatz function
  The Collatz function is defined as C(n) = n / 2 if n is even and C(n) = 3n + 1 if n is odd. The Collatz conjecture states that every sequence generated by the Collatz function ends with the cycle (4, 2, 1) after a finite number of iterations. In this paper it is shown that there exists no other cycle for the Collatz function. 
Manfred Bork
math.GM