I'm searching for some results about the uniqueness of equilibrium in a bimatrix game. In all articles that I can find the study is about existence of two matrix $A,B$ that have a given couple (x,y) as equilibrium. I'm interested in the viceversa: Given $(A,B)$ I'm interested in find if the game have one or more equilibrium point.
Thank you
EDIT: I've found in a book that the problem of unicity in bimatricial games is NP-Hard. Unfortunatly there isn't no more details, and no reference. Probabily it means that is necessary to find all nash equilibria to say if there is just one.