Marked PCP is decidable

We show that the marked version of the Post Correspondence Problem, where the words on a list are required to differ in the first letter, is decidable. On the other hand, we prove that the PCP remains undecidable if we only require the words to differ in the first two letters. Thus we locate the decidability/undecidability-boundary between marked and 2-marked PCP.

Authors

• Vesa Halava
• Mika Hirvensalo
• Ronald de Wolf

Keywords

• Post Correspondence Problem
• Marked morphism
• Decidability