An example of an automatic sequence with non-regular abelian complexity
This work addresses the open problem posed by Madill and Rampersad concerning the Abelian complexity of automatic sequences. By integrating techniques from formal language theory and combinatorics, the study investigates the structural properties and complexity measures of automatic sequences. The primary contribution is the construction of a 2-automatic sequence whose Abelian complexity is rigorously proven not to be 2-regular. This counterexample refutes the prevailing theoretical assumption that the Abelian complexity of automatic sequences is necessarily regular, thereby filling a significant gap in the existing literature. Ultimately, these findings provide essential theoretical foundations and offer new perspectives for future research on the complexity of automatic sequences.