# Stackelberg Equilibrium

In a Stackelberg model, a firm enter the market first. Its quantity anticipates the quantity that the other firm will produce.

## Example

The demand for coffee beans follows the inverse demand P = 1400 - 2 Q_Z - 2 Q_Y.

Zach chooses his quantity Q_Z first, then Yann follows.

## Yann's Best Response Function

When Yann enters the market, he chooses the best response to Q_Z.

Yann's revenue is R\left(Q_Y\right) = PQ = \left(1400 - 2 Q_Z - 2 Q_Y\right) Q_Y.

Yann's marginal revenue is MR\left(Q_Y\right) = 1400 - 2Q_Z - 4Q_Y.

Yann's marginal cost is MC\left(Q_Y\right) = 200.

Yann maximizes profit when

\begin{align*} MC\left(Q_Y\right) &= MR\left(Q_Y\right) \\ 200 &= 1400 - 2Q_Z - 4Q_Y \\ 4Q_Y &= 1200 - 2Q_Z \\ Q_Y &= 300 - \frac{Q_Z}{2} \end{align*}

## Zach's profit maximization problem

Zach's maximizes \Pi_Z \left(Q_Z \right) = \left(1400 - 2 Q_Z - 2 Q_Y\right) Q_Z -200 Q_Z

Plugging Q_Y = 300 - \frac{Q_Z}{2} gives

\begin{align*} \Pi_Z \left( Q_Z \right) &= \left(1400 - 2 Q_Z - 2 \left( 300 - \frac{Q_Z}{2} \right) \right) Q_Z - 200 Q_Z \\ &= \left(1400 - 2 Q_Z - 600 + Q_Z \right) Q_Z - 200 Q_Z \\ &= \left( 800 - Q_Z \right) Q_Z - 200 Q_Z \\ \end{align*}

Zach's profit is composed of a revenue R \left( Q_Z \right) = \left( 800 - Q_Z \right) Q_Z and a cost C \left( Q_Z \right) = 200 Q_Z.

When Zach maximizes profit

\begin{align*} MC \left( Q_Z \right) &= MR \left( Q_Z \right) \\ 200 &= 800 - 2 Q_Z \\ 2 Q_Z &= 600 \\ Q_Z &= 300 \end{align*}

Zach produces 300 coffee beans. Yann produces Q_Y = 300 - \frac{300}{2} = 150 coffee beans.

### Question

The inverse demand on the market for coffee is P = 288 - 8 ( Q_Z + Q_Y ).

Zach faces marginal costs equal to 240.

Yann faces marginal costs equal to 48.

What is the Stackelberg Equilibrium?

In equilibrium, Q_Z = -9.0 and Q_Y = 19.5.

Yann's marginal revenue is \begin{align*} MR (Q_Y) \begin{align*} MR ( Q_Y ) &= 288 - 8 Q_Z - 2 \times 8 Q_Y \\ &= 288 - 8 Q_Z - 16 Q_Y \end{align*} \end{align*} Yann's marginal cost is \begin{align*} MC (Q_Y) = 48 \end{align*} Yann's best response solves \begin{align*} MC ( Q_Y ) &= MR ( Q_Y ) \\ 48 &= 288 - 8 Q_Y - 16 Q_Z \\ 16 Q_Y &= 288 - 48 - 8 Q_Z \\ Q_Y &= \frac{288 - 48}{16} - \frac{Q_Z}{2} \\ Q_Y &= 15.0 - \frac{Q_Z}{2} \end{align*}
\begin{align*} &\Pi_Z = \left( 288 - 8 ( Q_Z + Q_Y ) \right) Q_Z - 240 Q_Z \\ &= \left( 288 - 8 Q_Z - 8 \left( 15.0 - \frac{Q_Z}{2} \right) \right) Q_Z - 240 Q_Z \\ &= \left( 288 - 8 Q_Z - 120.0 + 8 \frac{ Q_Z }{ 2 } \right) Q_Z - 240 Q_Z \\ &= \left( 168.0 - 8 \frac{ Q_Z }{ 2 } \right) Q_Z - 240 Q_Z \end{align*}

Maximizing Zach's profit give

\begin{align*} MC_Z &= MR_Z \\ 240 &= 168.0 - 8 Q_Z \\ 8 Q_Z &= 168.0 - 240 \\ 8 Q_Z &= -72.0 \\ Q_Z &= \frac{ -72.0 }{ 8 } \\ Q_Z &= -9.0 \end{align*}

Plug Q_Z = -9.0 into Yann's Best Response Function:

\begin{align*} Q_{ Y } &= 15.0 - \frac{Q_Z}{2} \\ &= 15.0 - \frac{ -9.0 }{2} \\ &= 15.0 - -4.5 \\ &= 19.5 \end{align*}