Suppose I wished to devise an algorithm (to be run on a turing machine) which could find the sum of 2 integers (expressed in base2). How many tapes would this require?

My intuition tells me that 4 tapes would be required: 1 for each of the two numbers being added together, 1 to store the 'carry over' value, and 1 to store the output.

Is this correct? Could it be done with fewer tapes?

  • 2
    $\begingroup$ It can be done with 1 tape. The tapes are infinite, right? $\endgroup$
    – mikeazo
    Nov 1, 2017 at 18:02

1 Answer 1


Anything that can be computed with any reasonable extension of a Turing machine can be computed with the standard Turing machine that has a single one-way infinite tape.

For example, you can simulate a four-tape machine by interleaving the characters of the tapes so that, e.g., the four tapes

$$\begin{array}{cccc} a_1 & a_2 & a_3 & \cdots\\ b_1 & b_2 & b_3 & \cdots\\ c_1 & c_2 & c_3 & \cdots\\ d_1 & d_2 & d_3 & \cdots \end{array}$$

with the single tape $$a_1\ b_1\ c_1\ d_1\ a_2\ b_2\ c_2\ d_2\ a_3\ b_3\ c_3\ d_3\ \cdots$$

  • $\begingroup$ Thanks for the answer. How would this machine work though? Would the $c_i$ terms not just be represented by different states of the head? $\endgroup$
    – M Smith
    Nov 2, 2017 at 0:38
  • $\begingroup$ I'm not sure what you mean. The values $c_1, c_2, \dots$ are the characters stored on the third tape. Actually, the situation is slightly more complicated than I stated, since you also need to keep track of where the tape heads are. $\endgroup$ Nov 2, 2017 at 1:15

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