# Prove that for every Nash equilibrium $\sigma^*$, the probability distribution $p_{\sigma^*}$ is a correlated equilibrium

This is a classic theorem in game theory, that is left as an excersice in my textbook. Can anybody proove it? I can not thing of anything excpet from the definition of the correlated equilibrium in first place. Here is the theorem and the definition as well.

$$\mathbf{Theorem:}$$ For every Nash equilibrium $$\sigma^*$$, the probability distribution $$p_{\sigma^*}$$ is a correlated equilibrium.

$$\mathbf{Definition:}$$ A probability distribution $$p$$ over the set of action vectors $$S$$ is called a correlated equilibrium if the strategy vector $$\tau^*$$ is a Nash equilibrium of the game $$\Gamma^*(p)$$. In other words, for every player $$i ∈ N$$:

$$$$\Sigma_{s_{-i}\in S_{-i}}p(s_i,s_{-i})u_i(s_i,s_{-i})\geq \Sigma_{s_{-i}\in S_{-i}}p(s_i,s_{-i})u_i(s^{'}_i,s_{-i}),\quad\text{\forall s_i,s^{'}_i\in S_i}$$$$

Every strategy vector $$\sigma$$ induces a probability distribution $$p_{\sigma^*}$$ over the set of action vector $$S$$. $$$$p_{\sigma^*}(s_1,...,s_n)=\sigma_1(s_1)\times\sigma_2(s_2)\times...\times\sigma_n(s_n)$$$$

$$\textit{Hint:}$$ When we relate to a Nash equilibrium $$\sigma^*$$ as a correlated equilibrium we mean the probability distribution $$p_{\sigma^*}$$ given by the aforementioned equation:

• @HerrK. how to doublecheck it? Sep 7, 2021 at 14:42
• Is there any typo? Where did you get the definition from? It seems inconsistent with the version given in the Fudenberg-Tirole textbook. Sep 7, 2021 at 14:52
• well it is from the Maschler, Solan and Zamir Game theory book, but from what I have seen until now, there are some typos in the book in general. However, I do not know abou the this one? @HerrK. can you write the Fundenberg - Tirole version as well to compare it? Sep 7, 2021 at 14:55
• @HerrK. I think that this version is the same with the equivalent version of the definition $2.4B$ in page 57 of Fundenberg - Tirole textbook...please check it if you want! Thank you in advance! Sep 7, 2021 at 15:13
• I now see where the difference is. Sorry for the confusion. Sep 7, 2021 at 15:23

A strategy profile $$\sigma^*=(\sigma_i^*,\sigma_{-i}^*)$$ is a Nash equilibrium if for all player $$i$$, $$$$u_i(s_i,\sigma_{-i}^*)\ge u_i(s_i',\sigma_{-i}^*), \quad \forall s_i\in\mathrm{supp}(\sigma_i^*), \forall s_i'\in S_i.$$$$ Rewriting this condition explicitly in terms of the probabilities over pure strategies: \begin{align} \sum_{s_{-i}\in S_{-i}}\sigma_{-i}^*(s_{-i})u_i(s_i,s_{-i}) &\ge \sum_{s_{-i}\in S_{-i}}\sigma_{-i}^*(s_{-i})u_i(s_i',s_{-i}), \quad \forall s_i\in\mathrm{supp}(\sigma_i^*), \forall s_i'\in S_i. \tag{1} \end{align} Let $$p_{\sigma^*}(s_i,s_{-i})=\sigma_i^*(s_i)\sigma_{-i}^*(s_{-i})$$ be the joint distribution implied by the equilibrium strategies. Then condition $$(1)$$ can be rewritten as $$$$\sum_{s_{-i}\in S_{-i}}\underbrace{\sigma_i^*(s_i)\sigma_{-i}^*(s_{-i})}_{p_{\sigma^*}(s_i,s_{-i})}u_i(s_i,s_{-i}) \ge \sum_{s_{-i}\in S_{-i}}\underbrace{\sigma_i^*(s_i)\sigma_{-i}^*(s_{-i})}_{p_{\sigma^*}(s_i,s_{-i})}u_i(s_i',s_{-i}), \quad\forall s_i,s_i'\in S_i.$$$$ This is the definition of correlated equilibrium.
• I am a bit confused...why we sum only with respect to $\sigma^*_{-i}(s_{-i})$ in the condition $(1)$ and then we chacnge this? I am missing something here.... Sep 7, 2021 at 15:28
• Did you mean why $u_i(s_i,\sigma_i)=\sum_{s_{-i}\in S_{-i}} \sigma_{-i}(s_{-i})u_i(s_i,s_{-i})$? Sep 7, 2021 at 15:32
• Well, $u_i(s_i,\sigma_{-i})$ is $i$'s expected utility from playing pure strategy $s_i$ (i.e. with probability $\color{red}{1}$) against the mixed strategy profile $\sigma_{-i}$, so his payoff would be $(\color{red}{1})\sigma_{-i}(s_{-i})u_i(s_i,s_{-i})$. If $i$ were to play a mixed strategy $\sigma_i$, then his expected payoff would be $\color{red}{\sigma_i(s_i)}\sigma_{-i}(s_{-i})u_i(s_i,s_{-i})$ Sep 7, 2021 at 15:38
• Ok I understand now. One last thing. Is it $u_i(s_i,\sigma_i)=\sum_{s_{-i}\in S_{-i}}\sigma_{-i}(s_{-i})u_i(s_i,s_{-i})$ or $u_i(s_i,\sigma_{-i})=\sum_{s_{-i}\in S_{-i}}\sigma_{-i}(s_{-i})u_i(s_i,s_{-i})$? Sep 7, 2021 at 15:43