Within analysis of perfect competition, we distinguish between the short run and the long run on the basis that use of some input factors is fixed in the short run, but variable in the long run.
Assume that all firms in an industry have access to the same technology of production. Using diagrams, demonstrate the relationship between the short run and long run total cost functions, and between the short run and long run average and marginal cost functions. Using an example in which the total cost is a cubic function, show how the most common diagrammatic explanation of profit maximisation in textbooks requires an assumption of eventually diminishing returns to scale, and discuss the plausibility of that assumption.