Consider a simple random walk on the four vertex graph (Square shape with A,B,C,D being vertex)
Assume that the payoff function is: $$f(A) =2,\\ f(B) = 4,\\ f(C) = 5,\\ f(D) = 3.$$ Assume that there is no cost associate with moving, but there is a discount factor $a$. What is the largest possible value of a so that the optimal stopping strategy is to stop at every vertex, i.e., so that $S_2 = \{A,B,C,D\}$?
I have really hard time solving this problem. Please help me.