一句话:证明若下推系统互模拟等价于有限系统,则存在规模初等有界的有限等价系统,从而该判定问题是初等可判定的。
论文研究下推系统的互模拟有限性问题:给定下推系统,判定它是否与某个有限系统互模拟等价。此前该问题已知可判定,但最好上界为 Ackermann 级。作者证明了一个更强的定量结果:若下推系统与有限系统互模拟等价,则已存在一个与其互模拟等价的有限系统,其规模在原下推系统描述规模上是初等有界的。作为推论,互模拟有限性判定问题是初等可判定的,从而将此前最优的 Ackermann 上界显著改进为初等上界。
We show that in case a pushdown system is bisimulation equivalent to a finite system, there is already a bisimulation equivalent finite system whose size is elementarily bounded in the description size of the pushdown system. As a consequence we obtain that it is elementarily decidable if a given pushdown system is bisimulation equivalent to some finite system. This improves a previously best-known ACKERMANN upper bound for this problem.