1 research outputs found
One pebble versus log n bits
We show that pebble machines (i.e., 2-way nondeterministic Turing machines with one pebble that can be moved onto the input cells) working in sublogarithmic weak space are not able to simulate strong log-space bounded 2-way deterministic Turing machines. We prove this by exhibiting a witness language. Moreover, we provide an optimal lower bound on the product of space and input head reversals for pebble machines accepting nonregular languages