Elementariness of a finite set of words is co-NP-complete

@article{Nraud1990ElementarinessOA,
  title={Elementariness of a finite set of words is co-NP-complete},
  author={Jean N{\'e}raud},
  journal={ITA},
  year={1990},
  volume={24},
  pages={459-470}
}