I recently took ECON 325 with Basyl Golovedstkyy (SFU) for my summer semester. It’s a fairly comprehensive course focusing on Industrial Organization modelling how firms can behave when they have market power but aren’t completely in a monopolistic setting.

One of the problems I had an issue on personally on the final exam was this model of a Cournot oligopoly with a variable amount (n) of firms. Being so frustrated with myself, I decided to go over it again slowly now that my semester is over because I know that it certainly will pop up in the future.

If you’re looking for a clear explanation of the model, look no further! Now, let’s begin.

# A Cournot-Style Oligopoly

## Model Setup

Let’s keep things fairly general.

**Market inverse demand curve:**

**Cost function: **

We know in a Cournot model that at equilibrium, all firms will produce exactly the same amount . Therefore, the sum of all those individual yet **homogeneous** outputs will equal :

Armed with this knowledge, let’s expand our inverse demand function by substituting with :

Let’s factor out one since we’ll be solving for one.

Hmm, seems like a good choice.

Now, to simplify a little, let us pause and consider . What exactly is this?

It’s actually with one of the taken out. This means we can write that as:

This allows us to rewrite and greatly simplify our demand function:

## Marginal Revenue & Costs

To get we must equate marginal revenue to marginal cost () and solve for .

Let’s find our marginal revenues and costs: the derivatives of our demand and cost functions with respect to .

We know that marginal cost is simply the derivative of the cost function we are given with respect to the we need.

Taking the derivative and simplifying further (we can also use our knowledge that marginal revenue is simply the inverse demand with double the slope for the in question):

## Profit Maximization

Now that we have both marginal cost and revenue, we can now profit maximize and find .

We know so let’s substitute it in our equation above:

Solving for :

Now, we know that . Let’s substitute this for .

And again, since we know :

Now that we have total market output at equilibrium, we can deduce what equilibrium price will be in the market.

From hereon in, you can go ahead and find the revenues or loses, equilibrium profits for each firm or the total profit the industry makes.

If this helped, please leave a comment below.

If you really feel like I’ve saved you with this, you can send me some of your hard earned money with PayPal or Kofi. That would super be cool!