Computational Complexity of Synchronization under Sparse Regular Constraints Stefan Hoffmannr0000´0002´7866´075Xs Informatikwissenschaften, FB IV, Universit¨atTrier, Germany,
[email protected] Abstract. The constrained synchronization problem (CSP) asks for a synchronizing word of a given input automaton contained in a regular set of constraints. It could be viewed as a special case of synchronization of a discrete event system under supervisory control. Here, we study the computational complexity of this problem for the class of sparse regular constraint languages. We give a new characterization of sparse regular sets, which equal the bounded regular sets, and derive a full classification of the computational complexity of CSP for letter-bounded regular constraint languages, which properly contain the strictly bounded regular languages. Then, we introduce strongly self-synchronizing codes and investigate CSP for bounded languages induced by these codes. With our previous result, we deduce a full classification for these languages as well. In both cases, depending on the constraint language, our problem becomes NP-complete or polynomial time solvable. Keywords: automata theory · constrained synchronization · compu- tational complexity · sparse languages · bounded languages · strongly self-synchronizing codes 1 Introduction A deterministic semi-automaton is synchronizing if it admits a reset word, i.e., a word which leads to a definite state, regardless of the starting state. This notion has a wide range of applications, from software testing, circuit synthe- sis, communication engineering and the like, see [13,45,47]. The famous Cern´yˇ conjecture [11] states that a minimal synchronizing word, for an n state automa- ton, has length at most pn ´ 1q2.