(Tragedy of the commons) Imagine that two fishermen operate in a lake. We denote the quantity fished by fisherman 1 as q1 and the quantity fished by fisherman 2 as q2. Each player can sell each unit of fish they get by the constant market price p (taken as given). However, the more they fish, the more difficult it gets to fish additional units, as the fish in the lake gets scarcer. So we assume that the cost from player i from fishing qi units, given that the other player fishes q−i units is given by qiq−i + q^2 i .