Let $\mathsf{ZPL}$/$\mathsf{RL}$/$\mathsf{BPL}$ denote the classes of the languages which are accepted (with zero/one-side/two-side error) by a logspace Turing machine with one-way access to the random tape.

Let $\mathsf{ZP^*L}$/$\mathsf{R^*L}$/$\mathsf{BP^*L}$ denote the corresponding classes by replacing one-way to two-way access. We have the normal inclusions $\mathsf{ZPL} \subseteq\mathsf{RL}\subseteq\mathsf{BPL}$ and $\mathsf{ZP^*L} \subseteq\mathsf{R^*L}\subseteq\mathsf{BP^*L}$.

In the following paper,

On Read-Once vs. Multiple Access to Randomness in Logspace, Noam Nisan, 1995

he prove that $\mathsf{BPL} \subseteq \mathsf{ZP^*L}$, which is rather surprising because it seems that two-way access may be stronger than the one-way access.

Question. Do we have any further results which demonstrate the power of two-way access randomized logspace?


1 Answer 1


Well, here are a couple of observations. There's a famous PRG by Nisan that fools $\mathsf{BPL}$-type algorithms with seed length $O(\log^2 n)$. Given two-way access to the seed, Nisan's PRG can be computed in space $O(\log n)$. Therefore, every language in $\mathsf{BPL}$ can be decided by a $\mathsf{BP}^*\mathsf{L}$-type algorithm that only uses $O(\log^2 n)$ random bits.

In fact, you can get the best of both worlds. By using the same technique that Nisan used to show $\mathsf{RL} \subseteq \mathsf{SC}$, one can show that every language in $\mathsf{BPL}$ can be decided by a $\mathsf{ZP}^*\mathsf{L}$-type algorithm that only uses $O(\log^2 n)$ random bits.

So in terms of the state of the art, two-way randomness is more powerful both qualitatively (two-sided error vs. zero-sided error) and quantitatively (polynomially many random bits vs. polylogarithmically many random bits). However, it seems likely that $\mathsf{L} = \mathsf{BP}^*\mathsf{L}$ and hence the two models are actually equivalent.


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