Gairola, U. C., and R. Krishan. “Coincidence and Fixed Point Theorems for Multivalued and Singlevalued Mappings”. The Journal of the Indian Mathematical Society, vol. 82, no. 3-4, Dec. 2015, pp. 65-82, https://www.informaticsjournals.com/index.php/jims/article/view/1683.