There are many instances in which there exist "Similar" products that are close سلعة بديلة (such as butter and margarine), which are relatively easily interchangeable[لغات أخرى], such that a rise in the price of one good will cause a significant shift to the consumption of the سلعة بديلة. If the cost of changing a firm's manufacturing process to produce the سلعة بديلة is also relatively "immaterial" in relationship to the firm's overall profit and cost, this is sufficient to ensure an economic situation isn't significantly different from a Perfectly Competitive economic سوق.
Roger LeRoy Miller, "Intermediate Microeconomics Theory Issues Applications, Third Edition", New York: McGraw-Hill, Inc, 1982.
Edwin Mansfield, "Micro-Economics Theory and Applications, 3rd Edition", New York and London:W.W. Norton and Company, 1979.
Henderson, James M., and Richard E. Quandt, "Micro Economic Theory, A Mathematical Approach. 3rd Edition", New York: McGraw-Hill Book Company, 1980. Glenview, Illinois: Scott, Foresmand and Company, 1988.
John Black, "Oxford Dictionary of Economics", New York: Oxford University Press, 2003.
Tirole, Jean, "The Theory of Industrial Organization", Cambridge, Massachusetts: The MIT Press, 1988.
