Remarks on separating words
  The separating words problem asks for the size of the smallest DFA needed to distinguish between two words of length <= n (by accepting one and rejecting the other). In this paper we survey what is known and unknown about the problem, consider some variations, and prove several new results. 
Erik D. Demaine, Sarah Eisenstat, Jeffrey Shallit, David A. Wilson
cs.FL cs.DM