Syntactic complexity of bifix-free languages.

We study the properties of syntactic monoids of bifix-free regular languages. In particular, we solve an open problem concerning syntactic complexity: We prove that the cardinality of the syntactic semigroup of a bifix-free language with state complexity n is at most ((n-1)^{n-3}+(n-2)^{n-3}+(n-3)2^{n-3}) for (n geqslant 6). The main proof uses a large construction with the method of injective function. Since this bound is known to be reachable, and the values for (n leqslant 5) are known, this completely settles the problem. We also prove that ((n-2)^{n-3} + (n-3)2^{n-3} - 1) is the minimal size of the alphabet required to meet the bound for (n geqslant 6). Finally, we show that the largest transition semigroups of minimal DFAs which recognize bifix-free languages are unique up to renaming the states.