I would like to know what exactly are the functions of the form $$ \pi(x, y) = P(x+y)x - C(x), $$ i.e., the profit functions arising from symmetric Cournot duopolies with inverse demand functions $P$ and cost functions $C$, where $P$ and $C$ satisfy very mild conditions ($P$ is nonincreasing where its values are positive, and $C$ is nondecreasing, but I want to assume as few analytic properties as possible).
Existence of Cournot equilibria has been studied under very general settings, and I imagined that those studies include answers to my question, but going through some works by Amir, McManus and Vivek didn't lead me to a success.
Has the class of functions of the form above been characterized in the literature for some mild assumptions on $P$ and $C$?