# Tag Info

Accepted

### What are the obstructions to extending $L=SL$ to $L=NL$?

The central problem is that, on directed graphs, even a truly random walk doesn't hit all the vertices in expected polynomial time, let alone a pseudorandom walk. The standard counterexample here is ...
Accepted

### What are the consequences of solving XOR 3-SAT in Logspace?

Take a look at https://www.sciencedirect.com/science/article/pii/S0022000008001141 "The complexity of satisfiability problems: Refining Schaefer's theorem" by Allender et al. which answer your ...

### $BPL$ with polylog random bits is in $L$

It follows from this PRG of Nisan and Zuckerman. This paper shows that if you have an algorithm that uses space $S$ and only $\mathrm{poly}(S)$ random bits, then the number of random bits can be ...
Accepted

### On sparse complete sets and P vs L

Yes, exactly what you suggested is true: if there is a sparse $\mathbf{P}$-complete set under log-space many-one reductions, then $\mathbf{P} = \mathbf{L}$. This was conjectured by Hartmanis in 1978 ...

### k-Vertex Cover problem is in parameterized Log space

Here is an algorithm that uses $2k^2 + O(\log n)$ space. This is just the observation that the well known "Buss kernel" for Vertex Cover can be computed in log-space: Say that a vertex has big degree ...
Accepted

### What are the consequences of $P \subseteq L/poly$?

One simple consequence is $\mathbf{P}/\text{poly} = \mathbf{L}/\text{poly}$. Proof: For any language $A \in \mathbf{P}/\text{poly}$, there is a language $B \in \mathbf{P}$ and a sequence of polynomial-...
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### Is Circuit Minimization $P$-hard under logspace reductions?

The Circuit Minimization Problem you describe is also known as MCSP (the Minimum Circuit Size Problem), which has been the subject of quite a few papers recently. (I'm posting an answer to this ...
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### It is known that $L \subsetneq PH$?

This is equivalent to $LOGSPACE≠NP$ (which is obviously open). The proof of that equivalence relativizes (at least under the usual oracle models). And there are oracles making $LOGSPACE = NP$ (the ...
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### What circuit depth is enough to compute a log-space complete problem?

It seems that nobody has added to the discussion of this question since February. I'm quite sure that no better depth upper bound is known for L than $\log^2 n$, in the bounded fan-in circuit model, ...

### NFA to 2DFA: what are the upper and lower bounds?

The recent survey Two-Way Finite Automata: Old and Recent Results by Pighizzini states in the introduction: The costs of the simulations of 1NFAs by 2DFAs and of 2NFAs by 2DFAs are still unknown. ...

### A super-linear time problem in NL

I believe that the "state of the art" is that it is extremely hard to prove superlinear lower bounds even for NP-complete problems. If you're fine with conditional lower bounds, then the ...

### Complexity of comparison unary>binary

The problem is in coNLOGTIME, for example using the following algorithm. As is well known, one can determine the length of input $n$ in binary in DLOGTIME. Then, read off at most $\log n$ bits from ...
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### Why NL is not L

You are right in noticing that the state space of an NL machine is only polynomially large (i.e. the number of reachable states is polynomial in the length $n$ of the input). A deterministic Logspace ...
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### Can $L=SL$ be shown with the replacement product instead of the zig-zag product?

There is a paper already in 2005 that describes how to do this... See https://people.seas.harvard.edu/~salil/research/derand_squaring-abs.html I cannot say why people use zig-zag instead, other than ...
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### Closure properties of $L$ (DLOGSPACE)?

This question is not research level, even so showing the equivalence of closure under kleene-star to the well known open problem L=NL was a nice challenge. Obviously $S\cap T$ and $S.T$ are in ...