Translational lemmas for DLOGTIME-uniform circuits, alternating TMs, and PRAMs

Abstract

We present translational lemmas for the three standard models of parallel computation, and apply them to obtain tight hierarchy results. It is shown that, for arbitrarily small rational constant $\epsilon > 0$ , (i) there is a language which can be accepted by a $U_{\rm E}$ -uniform circuit family of depth $c(1+\epsilon)(\log n)^{r_1}$ and size $dn^{r_2(1+\epsilon)}$ but not by any $U_{\rm E}$ -uniform circuit family of depth $c(\log n)^{r_1}$ and size $dn^{r_2}$ , (ii) there is a language which can be accepted by a $c(9+\epsilon)(\log n)^{r_1}$ -time $d(4+\epsilon)\log n$-space ATM with l worktapes but not by any $c(\log n)^{r_1}$ -time $d\log n$ -space ATM with the same l worktapes if the number of tape symbols is fixed, and (iii) there is a language which can be accepted by a $c(1+\epsilon)(\log n)^{r_1}$ -time PRAM with $dn^{r_2(1+\epsilon)}$ processors but not by any $c(\log n)^{r_1}$ -time PRAM with $dn^{r_2}$ processors. Here, c > 0, d ≥ 1, r ₁ > 1, and r ₂ ≥ 1 are arbitrary rational constants, and l ≥ 2 is an arbitrary integer.