Heikkila, Seppo. “Fixed Point Results for Set-Valued and Single-Valued Mappings in Ordered Spaces”. CUBO, A Mathematical Journal 10, no. 4 (December 1, 2008): 119–135. Accessed May 17, 2022. https://revistas.ufro.cl/ojs/index.php/cubo/article/view/1493.