Popa, Valeriu. “Inter-Relations Among Multifunctions Defined on Bitopological Spaces”. Rev. Anal. Numér. Théor. Approx. 21, no. 1 (February 1, 1992): 69–74. Accessed May 9, 2024. https://ictp.acad.ro/jnaat/journal/article/view/1992-vol21-no1-art10.