Budaghyan, L.E., et al. “A NECESSARY AND SUFFICIENT CONDITION FOR THE UNIQUENESS OF $ \beta\delta $-NORMAL FORM OF TYPED $ \lambda $-TERMS FOR THE CANONICAL NOTION OF $ \delta $-REDUCTION”. Proceedings of the YSU A: Physical and Mathematical Sciences, vol. 53, no. 1 (248), Apr. 2019, pp. 28-36, doi:10.46991/PYSU:A/2019.53.1.028.