Between SC and LOGDCFL: Families of Languages Accepted by Logarithmic-Space Deterministic Auxiliary Depth-k Storage Automata

TL;DR

This paper introduces a new class of automata called depth-$k$ storage automata, explores their computational power, and situates their language families between known complexity classes, extending classical results and providing new characterizations.

Contribution

It defines the $k$-storage automata model, studies the closure properties of their language families, and characterizes these families in terms of polynomial-time machines and universal simulators.

Findings

$ ext{DCFL} ext{ is contained in } k ext{SDA}$

$ ext{LOG}k ext{SDA}$ lies between $ ext{LOGDCFL}$ and $ ext{SC}$

A $ ext{LOG}k ext{SDA}$-complete language is constructed

Abstract

The closure of deterministic context-free languages under logarithmic-space many-one reductions ($\mathrm{L}$-m-reductions), known as LOGDCFL, has been studied in depth from an aspect of parallel computability because it is nicely situated between $\mathrm{L}$ and $\mathrm{AC}^{1}\cap\mathrm{SC}^2$. By replacing a memory device from pushdown stacks with access-controlled storage tapes, we introduce a computational model of one-way deterministic depth-$k$ storage automata ($k$-sda's) whose tape cells are freely modified during the first $k$ accesses and then become blank forever. These $k$-sda's naturally induce the language family $k\mathrm{SDA}$. Similarly to $\mathrm{LOGDCFL}$, we study the closure $\mathrm{LOG}k\mathrm{SDA}$ of all languages in $k\mathrm{SDA}$ under $\mathrm{L}$-m-reductions. We demonstrate that $\mathrm{DCFL}\subseteq k\mathrm{SDA}\subseteq \mathrm{SC}^k$ by significantly extending Cook's early result (1979) of $\mathrm{DCFL}\subseteq \mathrm{SC}^2$. The entire hierarch of $\mathrm{LOG}k\mathrm{SDA}$ for all $k\geq1$ therefore lies between $\mathrm{LOGDCFL}$ and $\mathrm{SC}$. As an immediate consequence, we obtain the same simulation bounds for Hibbard's limited automata. We further characterize $\mathrm{LOG}k\mathrm{SDA}$ in terms of a new machine model, called logarithmic-space deterministic auxiliary depth-$k$ storage automata that run in polynomial time. These machines are as powerful as a polynomial-time two-way multi-head deterministic depth-$k$ storage automata. We also provide a ``generic'' $\mathrm{LOG}k\mathrm{SDA}$-complete language under $\mathrm{L}$-m-reductions by constructing a two-way universal simulator working for all $k$-sda's.