η-Relations and Transitivity Conditions of η on Hyper-MV Algebras. - Saeed Rasouli; D. Heidari; Bijan Davvaz (J. Multiple Valued Log. Soft Comput., 2009)

Characterization of small polygroups by their fundamental groups. - D. Heidari; Bijan Davvaz (Discret. Math. Algorithms Appl., 2019)