 Nash equilibrium and definability The main solution concept to analyze strategic games is Nash equilibrium. Informally a Nash equilibrium is a collection of strategies, one per player, such that no player is interested to change his or her strategy, given the others stick to theirs. Formally, a strategy profile \(\sigma\) is a (pure strategy) Nash equilibrium if for all players \(i\in N\) and for all \(\sigma'_i\in \Sigma_i\) we have that