Caldas, M., E. Hatir, S. Jafari, and T. Noiri. “A New Kupka Type Continuity, λ-Compactness and Multifunctions”. CUBO, A Mathematical Journal, vol. 11, no. 4, Sept. 2009, pp. 1–13, https://cubo.ufro.cl/ojs/index.php/cubo/article/view/1449.