Lekkoksung, Nareupanat, and Prakit Jampachon. “IDEMPOTENT ELEMENTS OF PRE-GENERALIZED HYPERSUBSTITUTIONS OF TYPE (m, N)”. Italian Journal of Pure and Applied Mathematics, vol. 36, July 2016, pp. 827-42, https://journals.uniurb.it/index.php/ijpam/article/view/6481.