Authors: Christian Choffrut ¹; Robert Mercas ^2,3

• 1 Laboratoire d'informatique Algorithmique : Fondements et Applications
• 2 Research Group on Mathematical Linguistics
• 3 Departament de Filologies Romàniques [Tarragona]

We consider the monoid T with the presentation < a, b; aab = aba > which is ''close'' to trace monoids. We prove two different types of results. First, we give a combinatorial description of the lexicographically minimum and maximum representatives of their congruence classes in the free monoid \a, b\* and solve the classical equations, such as commutation and conjugacy in T. Then we study the closure properties of the two subfamilies of the rational subsets of T whose lexicographically minimum and maximum cross-sections respectively, are rational in \a, b\*.