Binary equality sets are generated by two words
arXiv:1209.4061 · doi:10.1016/S0021-8693(02)00534-3
Abstract
We show that the equality language of two non-periodic binary morphisms is generated by at most two words. If its rank is two, then the generators start (and end) with different letters. This in particular implies that any binary language has a test set of cardinality at most two.
Thoroughly revised version of a journal publication