Right Turing machine

Question

I asked to check if Turing machine that can move only right (or stay) is equal to a standard Turing machine .

I thought to copy the input to another tape, which unrestricted. but is it possible?

thank u.

Answer 1

Consider such a TM that always terminates, has n states and tape/input alphabet of {0,1}. On an input of size m, it must halt after at most 2*m*n steps. That's because it cannot go through the same state reading the same symbol twice without advancing; if it did, it would not halt.

This means all problems solvable by such TM are in P. On the other hand, regular TMs exists for solving problems in EXPTIME. Since P is a proper subset of EXPTIME, the two models are not equivalent.

BTW: A Turing machine usually only has one tape, so copying to a different tape is not an opion.

Answer 2

A turing machine that could move only right and stay is a variation of the Turing machine and is a subset of the standard turing machine. So essentially it is not as powerful as the standard turing machine but is a TM nonetheless.

Furthermore, combining multiple turing machine tapes does NOT give you more computing power and is also completely equivalent to a single standard TM.

(Multiple tapes are only advantageous in terms of efficiency, not computation. But since TMs not concerned about efficiency but only computability, this is irrelevant).