A Factor-Two Lower Bound for Complementing Two-Way Deterministic Finite Automata

Abstract

For every $n\ge22$, we construct a language over a fixed three-letter alphabet whose two-way deterministic state complexity is $n$ and whose complement has state complexity $2n$. Acceptance is at the left endmarker, and infinite computations reject. The lower bound holds for arbitrary deterministic recognizers. The proof uses the group of transformations induced by the witness to constrain the behavior of a smaller recognizer, then derives a contradiction from a permutation query problem. A matching halting complement follows from a $2n$-state construction for sources whose destination states determine the direction of each transition. Splitting states by incoming direction gives a $2n+2b$ upper bound for general sources, where $b$ counts states entered in both directions. In the acceptance-anywhere model of Geffert, Mereghetti, and Pighizzini, the same family has exact complexities $N$ and $2N-1$ for every $N\ge23$.