# Is there any research, theory, or anything which shows how much of a market has to be in power of a few companies to be a force against free market?

According to Free market

a free market is a system in which the prices for goods and services are determined by the open market and by consumers. In a free market, the laws and forces of supply and demand are free from any intervention by a government or other authority and from all forms of economic privilege, monopolies and artificial scarcities

Being monopolies and oligopolies a force against free market, is there any study, theory or anything which shows how much of a market has to be in power of a few companies for them to be able to alter the free market laws/rules?

• Are you aware of the Herfindahl-Hirschman Index? – Herr K. Jul 25 '19 at 21:55
• @ Herr K. HHI is just an accounting mesure as far as market power is concerned it doesn't say much. That is why Competition Authorities only use it as a scrrening device. For example if a market is open to "hit-and-run" operations even a monopolist cannot charge much above marginal costs. – Grada Gukovic Jul 26 '19 at 13:06

Let's look at the cournot model of competition. There are $$n$$ firms, each with idiosyncratic marginal cost $$c_i$$. Each firm simultaneously chooses its output (supply) quantity, $$q_i$$. Given consumers' demand, this results in some market price $$P(Q)$$, where $$Q=\sum_{i=1}^n q_i$$ is the total industry output. We would expect the demand curve to be downward sloping: $$P'(Q)<0$$.
Thus, a firm's profits are $$\pi_i(q_i,\mathbf{q}_{-i})=\left(P(Q)-c_i\right)q_i.$$ The first-order condition for maximisation of profit is $$\frac{\partial\pi_i}{\partial q_i}=P(Q)-c_i+\frac{\partial P(Q)}{\partial q_i}=0.$$ Noting that $$\frac{\partial P(Q)}{\partial q_i}=\frac{\partial P(Q)}{\partial Q},$$ the first-order condition can be re-written as $$\frac{P(Q)-c_i}{c_i}=-\frac{\partial P(Q)}{\partial Q}\frac{Q}{P(Q)}\frac{q_i}{Q},$$ $$\frac{P(Q)-c_i}{c_i}=-\frac{1}{\eta}m_i,$$ where $$\eta$$ is the price elasticity of demand and $$m_i=q_i/Q$$ is firm $$i$$'s market share.
$$\text{avg market power}=\sum_i m_i \frac{P(Q)-c_i}{c_i}=-\frac{\sum_i m_i^2}{\eta}.$$
Note that $$\sum_i m_i^2$$ is the HHI index for the market. So this equation tells us that the average market power in an industry will be positively related to the HHI. For any positive HHI, there will be a positive mark-up and the market will deviate from the perfectly competitive outcome in which artificial scarcities do not exist. Put differently, only if there is an infinite number of firms, each controling an infinitely small share of the industry do we obtain the textbook perfect competition outcome.