This is a HW question, so Im not expecting full solutions or anything, but would love some direction. Also English is not my first language, so I apologize in advance.
We define a new class of languages - VNL:
all languages L s.t there exists M - a DTM with:
- an input-read-only tape.
- a "witness" tape which is a "one-time" read-only and can be read only as stay/right (the tape head cant move left).
- a work tape which is read/write.
- Also there exists p a polynomial s.t. for every w $\in \{0,1\}^*$: $w\in L\ \iff\ \exists u\in\left\{ 0,1\right\} ^{p\left(\left|w\right|\right)}:\ M\left(w,u\right)=acc$
- M is of O(log(|w|)) space complexity (where w is the input).
Now we need to prove VNL=NL. I think I managed the VNL$\subseteq$NL direction:
Let L$\in$VNL. then there exists M a DTM as we described before.
I then define a NDTM - $M_L$ on input w: that runs M on w,u where u is randomly generated, bit by bit, everytime M wants to move right.
It only needs to store an index to make sure u is of the right size, and that takes log(p(|w|) space, which is still logarithmic space complexity, and also running M is logarithmic. I also explain why M meets the other demands (but as they are somewhat trivial, I don't write them here to keep this post a bit shorter).
I have two questions:
- I'm not entirely sure how to do the other direction (NL$\subseteq$VNL). What I tried so far was: take L$\in$NL. $M_L$ is the logarithmic NDTM for it. I define M (of our new type) to treat the "witness" tape as a series of configurations. Somehow I'm supposed to verify its a legitimate configurations series resulting in M accepting w. Perhaps draw 2 consecutive configurations to the work tape and somehow verify the the transition between them is legitimate? Would love help with that.
- For another section of the question we remove the demand that the witness tape is read only to the right (which means I can read it as many times as I want). We're supposed to determine to which class VNL equals now. I thought about NP, but encountered some difficulties during the proof. Would also love help with that.
Thank you!