If a preference set is non-convex, then some prices determine a budget-line that supports two separate optimal-baskets. For example, we can imagine that, for zoos, a lion costs as much as an eagle, and further that a zoo's budget suffices for one eagle or one lion. We can suppose also that a zoo-keeper views either animal as equally valuable. In this case, the zoo would purchase either one lion or one eagle. Of course, a contemporary zoo-keeper does not want to purchase half of an eagle and half of a lion. Thus, the zoo-keeper's preferences are non-convex: The zoo-keeper prefers having either animal to having any strictly convex combination of both.
When the consumer's preference set is non-convex, then (for some prices) the consumer's demand is not connected; A disconnected demand implies some discontinuous behavior by the consumer, as discussed by Harold Hotelling:
If indifference curves for purchases be thought of as possessing a wavy character, convex to the origin in some regions and concave in others, we are forced to the conclusion that it is only the portions convex to the origin that can be regarded as possessing any importance, since the others are essentially unobservable. They can be detected only by the discontinuities that may occur in demand with variation in price-ratios, leading to an abrupt jumping of a point of tangency across a chasm when the straight line is rotated. But, while such discontinuities may reveal the existence of chasms, they can never measure their depth. The concave portions of the indifference curves and their many-dimensional generalizations, if they exist, must forever remain in
unmeasurable obscurity.[12]
The difficulties of studying non-convex preferences were emphasized by Herman Wold[13] and again by Paul Samuelson, who wrote that non-convexities are "shrouded in eternal darkness ...",[14] according to Diewert.[15]
When convexity assumptions are violated, then many of the good properties of competitive markets need not hold: Thus, non-convexity is associated with market failures, where supply and demand differ or where market equilibria can be inefficient.[1]
Non-convex preferences were illuminated from 1959 to 1961 by a sequence of papers in The Journal of Political Economy (JPE). The main contributors were Michael Farrell,[16] Francis Bator,[17]Tjalling Koopmans,[18] and Jerome Rothenberg.[19] In particular, Rothenberg's paper discussed the approximate convexity of sums of non-convex sets.[20] These JPE-papers stimulated a paper by Lloyd Shapley and Martin Shubik, which considered convexified consumer-preferences and introduced the concept of an "approximate equilibrium".[21] The JPE-papers and the Shapley–Shubik paper influenced another notion of "quasi-equilibria", due to Robert Aumann.[22][23]
Non-convex sets have been incorporated in the theories of general economic equilibria.[24] These results are described in graduate-level textbooks in microeconomics,[25] general equilibrium theory,[26]game theory,[27]mathematical economics,[28]
and applied mathematics (for economists).[29] The Shapley–Folkman lemma establishes that non-convexities are compatible with approximate equilibria in markets with many consumers; these results also apply to production economies with many small firms.[30]
Non-convexity is important under oligopolies and especially monopolies.[8] Concerns with large producers exploiting market power initiated the literature on non-convex sets, when Piero Sraffa wrote about on firms with increasing returns to scale in 1926,[31] after which Harold Hotelling wrote about marginal cost pricing in 1938.[32] Both Sraffa and Hotelling illuminated the market power of producers without competitors, clearly stimulating a literature on the supply-side of the economy.[33]
The previously mentioned applications concern non-convexities in finite-dimensional vector spaces, where points represent commodity bundles. However, economists also consider dynamic problems of optimization over time, using the theories of differential equations, dynamic systems, stochastic processes, and functional analysis: Economists use the following optimization methods:
Economists have increasingly studied non-convex sets with nonsmooth analysis, which generalizes convex analysis. Convex analysis centers on convex sets and convex functions, for which it provides powerful ideas and clear results, but it is not adequate for the analysis of non-convexities, such as increasing returns to scale.[53] "Non-convexities in [both] production and consumption ... required mathematical tools that went beyond convexity, and further development had to await the invention of non-smooth calculus": For example, Clarke's differential calculus for Lipschitz continuous functions, which uses Rademacher's theorem and which is described by Rockafellar & Wets (1998)[54] and Mordukhovich (2006),[9] according to Khan (2008).[10]Brown (1995, pp. 1967–1968) harvtxt error: no target: CITEREFBrown1995 (help) wrote that the "major methodological innovation in the general equilibrium analysis of firms with pricing rules" was "the introduction of the methods of non-smooth analysis, as a [synthesis] of global analysis (differential topology) and [of] convex analysis." According to Brown (1995, p. 1966) harvtxt error: no target: CITEREFBrown1995 (help), "Non-smooth analysis extends the local approximation of manifolds by tangent planes [and extends] the analogous approximation of convex sets by tangent cones to sets" that can be non-smooth or non-convex.[11][55]
^ abSalanié, Bernard (2000). "7 Nonconvexities". Microeconomics of market failures (English translation of the (1998) French Microéconomie: Les défaillances du marché (Economica, Paris) ed.). Cambridge, MA: MIT Press. pp. 107–125. ISBN0-262-19443-0.
^ abcStarrett, David A. (1972). "Fundamental nonconvexities in the theory of externalities". Journal of Economic Theory. 4 (2): 180–199. doi:10.1016/0022-0531(72)90148-2. MR0449575.
^ abcPages 106, 110–137, 172, and 248: Baumol, William J.; Oates, Wallace E.; with contributions by V. S. Bawa and David F. Bradford (1988). "8 Detrimental externalities and nonconvexities in the production set". The Theory of environmental policy (Second ed.). Cambridge: Cambridge University Press. ISBN978-0-521-31112-0.
^ abMordukhovich, Boris S. (2006). "Chapter 8 Applications to economics". Variational analysis and generalized differentiation II: Applications. Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences]. Vol. 331. Springer. especially Section 8.5.3 "Enter nonconvexity" (and the remainder of the chapter), particularly page 495. ISBN978-3-540-25438-6. MR2191745.
^Pages 231 and 239 (Figure 10 a–b: Illustration of lemma 5 [page 240]): Wold, Herman (1943b). "A synthesis of pure demand analysis II". Skandinavisk Aktuarietidskrift [Scandinavian Actuarial Journal]. Vol. 26. pp. 220–263. MR0011939.
Exercise 45, page 146: Wold, Herman; Juréen, Lars (in association with Wold) (1953). "8 Some further applications of preference fields (pp. 129–148)". Demand analysis: A study in econometrics. Wiley publications in statistics. New York: John Wiley and Sons, Inc. Stockholm: Almqvist and Wiksell. MR0064385.
It will be noted that any point where the indifference curves are convex rather than concave cannot be observed in a competitive market. Such points are shrouded in eternal darkness—unless we make our consumer a monopsonist and let him choose between goods lying on a very convex "budget curve" (along which he is affecting the price of what he buys). In this monopsony case, we could still deduce the slope of the man's indifference curve from the slope of the observed constraint at the equilibrium point.
^Bator, Francis M. (October 1961a). "On convexity, efficiency, and markets". The Journal of Political Economy. 69 (5): 480–483. doi:10.1086/258540. JSTOR1828537. S2CID153979194. Bator, Francis M. (October 1961b). "On convexity, efficiency, and markets: Rejoinder". Journal of Political Economy. 69 (5): 489. doi:10.1086/258542. JSTOR1828539. S2CID154255876.
Koopmans (1961, p. 478) and others—for example, Farrell (1959, pp. 390–391) and Farrell (1961a, p. 484), Bator (1961a, pp. 482–483), Rothenberg (1960, p. 438), and Starr (1969, p. 26) harvtxt error: no target: CITEREFStarr1969 (help)—commented on Koopmans (1957, pp. 1–126, especially 9–16 [1.3 Summation of opportunity sets], 23–35 [1.6 Convex sets and the price implications of optimality], and 35–37 [1.7 The role of convexity assumptions in the analysis]):
Pages 52–55 with applications on pages 145–146, 152–153, and 274–275: Mas-Colell, Andreu (1985). "1.L Averages of sets". The Theory of General Economic Equilibrium: A Differentiable Approach. Econometric Society Monographs. Cambridge University Press. ISBN0-521-26514-2. MR1113262.
Theorem C(6) on page 37 and applications on pages 115-116, 122, and 168: Hildenbrand, Werner (1974). Core and equilibria of a large economy. Princeton studies in mathematical economics. Princeton, NJ: Princeton University Press. ISBN978-0-691-04189-6. MR0389160.
Page 628: Mas–Colell, Andreu; Whinston, Michael D.; Green, Jerry R. (1995). "17.1 Large economies and nonconvexities". Microeconomic theory. Oxford University Press. pp. 627–630. ISBN978-0-19-507340-9.
Ellickson (1994, p. xviii), and especially Chapter 7 "Walras meets Nash" (especially section 7.4 "Nonconvexity" pages 306–310 and 312, and also 328–329) and Chapter 8 "What is Competition?" (pages 347 and 352): Ellickson, Bryan (1994). Competitive equilibrium: Theory and applications. Cambridge University Press. ISBN978-0-521-31988-1.
^Theorem 1.6.5 on pages 24–25: Ichiishi, Tatsuro (1983). Game theory for economic analysis. Economic theory, econometrics, and mathematical economics. New York: Academic Press, Inc. [Harcourt Brace Jovanovich, Publishers]. ISBN0-12-370180-5. MR0700688.
^Cassels, J. W. S. (1981). "Appendix A Convex sets". Economics for mathematicians. London Mathematical Society lecture note series. Vol. 62. Cambridge, New York: Cambridge University Press. pp. 33–34 and 127. ISBN0-521-28614-X. MR0657578.
^Pages 93–94 (especially example 1.92), 143, 318–319, 375–377, and 416: Carter, Michael (2001). Foundations of mathematical economics. Cambridge, MA: MIT Press. ISBN0-262-53192-5. MR1865841.
Pages 47–48: Florenzano, Monique; Le Van, Cuong (2001). Finite dimensional convexity and optimization. Studies in economic theory. Vol. 13. in cooperation with Pascal Gourdel. Berlin: Springer-Verlag. doi:10.1007/978-3-642-56522-9. ISBN3-540-41516-5. MR1878374. S2CID117240618.
^Pages 5–7: Quinzii, Martine (1992). Increasing returns and efficiency (Revised translation of (1988) Rendements croissants et efficacité economique. Paris: Editions du Centre National de la Recherche Scientifique ed.). New York: Oxford University Press. ISBN0-19-506553-0.
^Starrett discusses non-convexities in his textbook on public economics (pages 33, 43, 48, 56, 70–72, 82, 147, and 234–236): Starrett, David A. (1988). Foundations of public economics. Cambridge economic handbooks. Cambridge: Cambridge University Press. ISBN9780521348010.
^Page 270: Drèze, Jacques H. (1987). "14 Investment under private ownership: Optimality, equilibrium and stability". In Drèze, J. H.location=Cambridge (ed.). Essays on economic decisions under uncertainty. Cambridge University Press. pp. 261–297. doi:10.1017/CBO9780511559464. ISBN0-521-26484-7. MR0926685. (Originally published as Drèze, Jacques H. (1974). "Investment under private ownership: Optimality, equilibrium and stability". In Drèze, J. H. (ed.). Allocation under Uncertainty: Equilibrium and Optimality. New York: Wiley. pp. 129–165.)
^Magille & Quinzii, Section 31 "Partnerships", p. 371) harvtxt error: no target: CITEREFMagilleQuinzii (help): Magill, Michael; Quinzii, Martine (1996). "6 Production in a finance economy". The Theory of incomplete markets. Cambridge, Massachusetts: MIT Press. pp. 329–425.
^Troutman, John L. (1996). With the assistance of William Hrusa (ed.). Variational calculus and optimal control: Optimization with elementary convexity. Undergraduate Texts in Mathematics (Second ed.). New York: Springer-Verlag. doi:10.1007/978-1-4612-0737-5. ISBN0-387-94511-3. MR1363262.
^Vinter, Richard (2000). Optimal control. Systems & Control: Foundations & Applications. Boston, MA: Birkhäuser Boston, Inc. ISBN0-8176-4075-4. MR1756410.
^Beckmann, Martin; Muth, Richard F. (1954). "On the solution to the fundamental equation of inventory theory". Cowles Commission Discussion Paper. 2116.
^Heal (1999, p. 4 in preprint): Heal, G. M. (1999). "Introduction"(PDF). The economics of increasing returns. The International Library of Critical Writings in Economics. Edward Elgar. ISBN978-1-85898-160-4. Retrieved 5 March 2011.
Green, Jerry; Heller, Walter P. (1981). "1 Mathematical analysis and convexity with applications to economics". In Arrow, Kenneth Joseph; Intriligator, Michael D. (eds.). Handbook of mathematical economics, Volume I. Handbooks in economics. Vol. 1. Amsterdam: North-Holland Publishing Co. pp. 15–52. doi:10.1016/S1573-4382(81)01005-9. ISBN0-444-86126-2. MR0634800.
Heal, G. M. (April 1998). The Economics of Increasing Returns(PDF). PaineWebber working paper series in money, economics, and finance. Columbia Business School. PW-97-20. Archived from the original(PDF) on 15 September 2015. Retrieved 5 March 2011.