Better Complexity Bounds for Cost Register Automata

Cost register automata (CRAs) are one-way finite automata whose transitions have the side effect that a register is set to the result of applying a state-dependent semiring operation to a pair of registers. Here it is shown that CRAs over the tropical semiring (N∪{∞},min,+) can simulate polynomial time computation, proving along the way that a naturally defined width-k circuit value problem over the tropical semiring is P-complete. Then the copyless variant of the CRA, requiring that semiring operations be applied to distinct registers, is shown no more powerful than NC1 when the semiring is (Z,+,×) or (Γ∗ ∪ {⊥},max, concat). This relates questions left open in recent work on the complexity of CRA-computable functions to long-standing class separation conjectures in complexity theory, such as NC versus P and NC1 versus GapNC1. 1998 ACM Subject Classification F.1.3 Complexity Measures and Classes