Perfect competition
Short-run:- super-normal profits at first, new entrants, lowers price.
Long-run:- new entrants and lower prices mean; lowers demand curve and lowers MR/AR as a result, so only normal profit made (costs covered).
Monopoly - making large, super-normal profits and not allocative or productive efficient.
Oligopoly - kinked demand curve - elastic at lower output, inelastic at higher output, market operates at agreed equilibirum, no gain or loss of market and little to choose between competitors.
Economies of scale - fixed costs spread over increasing output, i.e. £10,000 for a telecommunications mast, number of customers to start off with is 5000, cost of using service is £2, customer numbers rise to 10,000, reduces cost of using service to £1.
I've also looked at price discrimination!
Doogie