WORM-2DPDAs: An Extension to 2DPDAs that can be Simulated in Linear Time

We extend 2-way deterministic push-down automata (2DPDAs) with a write-once-readmany (WORM) store. We show that it allows linear time simulation by a variant of Cook’s construction. As an example we develop a linear time algorithm that recognizes the language {V V −1WW−1 | V, W ∈ (a | b)∗}, that by Aho, Hopcroft and Ullman is conjectured not to be recognizable by a 2DPDA. Thus we believe that the extension strictly increases the expressive power of 2DPDAs.