The Collatz sequence is formed by an iteration of numbers produced by the following rule: If is even, then replace by ; if is odd then replace by . This Demonstration implements an 8-state 3-color Turing machine that computes this sequence.
The machine works like this: Write an initial number in base 2 on the blank tape. The head of the machine is initially located over the least significant byte of the number.
If is even, the head erases the digit 0 and moves to the left, so that it lies over the less significant byte of .
If is odd, the machine performs the sum by adding the number with itself shifted one position to the left plus one. To do this, the head writes 0, saves the digit 0 and the residue 1 and then moves to the left. Now, the head repeatedly adds the current digit plus the saved digit plus the residue saved in the head and saves the residue of the operation and the current digit in the head, then moves to the left. When the addition is done, the head returns to the position of the least significant byte of .
The computation does not stop at . It keeps computing the periodic sequence 1, 2, 4, 1, …, which can be interpreted as stopping.