I have 2 symmetric players $A$ and $B$.

Each of them has 2 decision variables $x_i\in[0, \beta]$ and $y_i\in[0,1]$, where $i\in\{A,B\}$.

Their payoff functions are symmetric, i.e., if you swap the label $A$ and $B$, you get the other player's payoff function. And the payoff function is continuous in both $x$ and $y$.

My question is: for this type of game, does it always have symmetric pure strategy equilibrium? In general, what are the conditions to guarantee the existence of symmetric pure strategy equilibrium?

  • $\begingroup$ In general it does not even have a pure strategy equilibrium. I believe there are some results of existence of such equilibrium in supermodular games. $\endgroup$ Jun 3, 2020 at 19:54

1 Answer 1

  1. There is no guarantee that symmetric games have symmetric equilibria. See this paper for concrete examples.
  2. There is also no guarantee that symmetric games have pure-strategy equilibria. For example, the following game is symmetric and has no pure-strategy equilibria $$ \begin{array}{cccc} & \mathrm{a} & \mathrm{b} & \mathrm{c}\\ \mathrm{a} & 1,1 & 0,2 & 5,0\\ \mathrm{b} & 2,0 & 3,3 & 0,4\\ \mathrm{c} & 0,5 & 4,0 & 1,1\\ \end{array} $$

  3. There are two special kinds of games that are known to always have pure strategy equilibria: supermodular games and potential games.

  • 1
    $\begingroup$ The first has a symmetric equilibrium. Just not in pure strategies. $\endgroup$ Jun 3, 2020 at 20:23
  • $\begingroup$ Thank you, I will change the example. $\endgroup$ Jun 3, 2020 at 20:31

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