Now let's consider Firm 1 Low, Firm 2 High as a possible Nash equilibrium. We can easily see that, if Firm 1 believed Firm 2 would play High it would like to switch to High as well, hence the arrow. This is enough to tell us that this outcome isn't a Nash equilibrium, if even one player would change her strategy if the other did not the outcome can't be an equilibrium.

   As it turns out, if Firm 2 believed Firm 1 would play Low it would like to switch to High, as the arrow shows. This too would be sufficient evidence to convince us this isn't an equilibrium outcome.

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