In economics, a **budget set**, or the opportunity set facing a consumer, is the set of all possible consumption bundles that the consumer can afford taking as given the prices of commodities available to the consumer and the consumer's income. Let the number of commodities available to the consumer in an economy be finite and equal to . Thus, for commodity amounts , also known as consumption plans which should not exceed the income,^{ [1] } with associated prices and consumer income , the budget set is defined as

- ,

where the consumption set is taken to be . It is typically assumed that and , in which case is also known as the **Walrasian**, or **competitive**, budget set.

The budget set is bounded above by a -dimensional budget hyperplane characterized by the equation , which in the two-good case corresponds to the budget line. Graphically, the budget set is the subset of that contains all the consumption bundles that lie on or below the budget hyperplane.

Given the framework described above, it can be shown that Walrasian budget sets are convex and compact.

Other sources of wealth, including stocks, savings, pensions, profit shares, etc., are not included in the income described above. The income described above are also known as initial wealth.^{ [1] }

The demand set is the set that the consumer chooses to go with based on the preferences from the budget set.^{ [1] }

As a topic of economics, **utility** is used to model worth or value. Its usage has evolved significantly over time. The term was introduced initially as a measure of pleasure or happiness as part of the theory of utilitarianism by moral philosophers such as Jeremy Bentham and John Stuart Mill. The term has been adapted and reapplied within neoclassical economics, which dominates modern economic theory, as a **utility function** that represents a single consumer's preference ordering over a choice set but is not comparable across consumers. This concept of utility is personal and based on choice rather than on pleasure received, and so is specified more rigorously than the original concept but makes it less useful for ethical decisions.

In geometry, a **normal** is an object such as a line, ray, or vector that is perpendicular to a given object. For example, the **normal line** to a plane curve at a given point is the (infinite) line perpendicular to the tangent line to the curve at the point. A normal vector may have length one or its length may represent the curvature of the object ; its algebraic sign may indicate sides.

In mathematics, a **linear form** is a linear map from a vector space to its field of scalars.

The theory of **consumer choice** is the branch of microeconomics that relates preferences to consumption expenditures and to consumer demand curves. It analyzes how consumers maximize the desirability of their consumption as measured by their preferences subject to limitations on their expenditures, by maximizing utility subject to a consumer budget constraint.

In economics, a **budget constraint** represents all the combinations of goods and services that a consumer may purchase given current prices within his or her given income. Consumer theory uses the concepts of a budget constraint and a preference map as tools to examine the parameters of consumer choices. Both concepts have a ready graphical representation in the two-good case. The consumer can only purchase as much as their income will allow, hence they are constrained by their budget. The equation of a budget constraint is where P_x is the price of good X, and P_y is the price of good Y, and m = income.

The **rank–nullity theorem** is a theorem in linear algebra, which asserts that the dimension of the domain of a linear map is the sum of its rank and its *nullity*.

In economics, the **marginal rate of substitution** (**MRS**) is the rate at which a consumer can give up some amount of one good in exchange for another good while maintaining the same level of utility. At equilibrium consumption levels, marginal rates of substitution are identical. The marginal rate of substitution is one of the three factors from marginal productivity, the others being marginal rates of transformation and marginal productivity of a factor.

In mathematics, more specifically in multivariable calculus, the **implicit function theorem** is a tool that allows relations to be converted to functions of several real variables. It does so by representing the relation as the graph of a function. There may not be a single function whose graph can represent the entire relation, but there may be such a function on a restriction of the domain of the relation. The implicit function theorem gives a sufficient condition to ensure that there is such a function.

In economics, **nominal** value is measured in terms of money, whereas **real** value is measured against goods or services. A real value is one which has been adjusted for inflation, enabling comparison of quantities as if the prices of goods had not changed on average. Changes in value **in real terms** therefore exclude the effect of inflation. In contrast with a real value, a nominal value has not been adjusted for inflation, and so changes in nominal value reflect at least in part the effect of inflation.

Utility maximization was first developed by utilitarian philosophers Jeremy Bentham and John Stewart Mill. In microeconomics, the **utility maximization problem** is the problem consumers face: "How should I spend my money in order to maximize my utility?" It is a type of optimal decision problem. It consists of choosing how much of each available good or service to consume, taking into account a constraint on total spending (income), the prices of the goods and their preferences.

In microeconomics, a consumer's **Marshallian demand function** is the quantity he/she demands of a particular good as a function of its price, his/her income, and the prices of other goods, a more technical exposition of the standard demand function. It is a solution to the utility maximization problem of how the consumer can maximize his/her utility for given income and prices. A synonymous term is **uncompensated demand function**, because when the price rises the consumer is not compensated with higher nominal income for the fall in his/her real income, unlike in the Hicksian demand function. Thus the change in quantity demanded is a combination of a substitution effect and a wealth effect. Although Marshallian demand is in the context of partial equilibrium theory, it is sometimes called **Walrasian demand** as used in general equilibrium theory.

There are two **fundamental theorems of welfare economics**. The **first** states that in economic equilibrium, a set of complete markets, with complete information, and in perfect competition, will be Pareto optimal. The requirements for perfect competition are these:

- There are no externalities and each actor has perfect information.
- Firms and consumers take prices as given.

**Convex optimization** is a subfield of mathematical optimization that studies the problem of minimizing convex functions over convex sets. Many classes of convex optimization problems admit polynomial-time algorithms, whereas mathematical optimization is in general NP-hard.

**Revealed preference theory**, pioneered by economist Paul Anthony Samuelson in 1938, is a method of analyzing choices made by individuals, mostly used for comparing the influence of policies on consumer behavior. Revealed preference models assume that the preferences of consumers can be revealed by their purchasing habits.

In microeconomics, a consumer's **Hicksian demand function** or **compensated demand function** for a good is his quantity demanded as part of the solution to minimizing his expenditure on all goods while delivering a fixed level of utility. Essentially, a **Hicksian demand function** shows how an economic agent would react to the change in the price of a good, if the agent's income was compensated to guarantee the agent the same utility previous to the change in the price of the good—the agent will remain on the same indifference curve before and after the change in the price of the good. The function is named after John Hicks.

In mathematics, the **tautological bundle** is a vector bundle occurring over a Grassmannian in a natural tautological way: for a Grassmannian of -dimensional subspaces of , given a point in the Grassmannian corresponding to a -dimensional vector subspace , the fiber over is the subspace itself. In the case of projective space the tautological bundle is known as the **tautological line bundle.**

In economics and consumer theory, **quasilinear utility** functions are linear in one argument, generally the numeraire. Quasilinear preferences can be represented by the utility function where is strictly concave. A useful property of the quasilinear utility function is that the Marshallian/Walrasian demand for does not depend on wealth and is thus not subject to a wealth effect; The absence of a wealth effect simplifies analysis and makes quasilinear utility functions a common choice for modelling. Furthermore, when utility is quasilinear, compensating variation (CV), equivalent variation (EV), and consumer surplus are algebraically equivalent. In mechanism design, quasilinear utility ensures that agents can compensate each other with side payments.

**Projective space** plays a central role in algebraic geometry. The aim of this article is to define the notion in terms of abstract algebraic geometry and to describe some basic uses of projective space.

In theoretical economics, an **abstract economy** is a model that generalizes both the standard model of an exchange economy in microeconomics, and the standard model of a game in game theory. An *equilibrium* in an abstract economy generalizes both a Walrasian equilibrium in microeconomics, and a Nash equilibrium in game-theory.

In mathematics, a **quadric** or **quadric hypersurface** is the subspace of *N*-dimensional space defined by a polynomial equation of degree 2 over a field. Quadrics are fundamental examples in algebraic geometry. The theory is simplified by working in projective space rather than affine space. An example is the quadric surface

- 1 2 3 Böhm, Volker; Haller, Hans (2017), "Demand Theory",
*The New Palgrave Dictionary of Economics*, London: Palgrave Macmillan UK, pp. 1–14, doi:10.1057/978-1-349-95121-5_539-2, ISBN 978-1-349-95121-5 , retrieved 2021-12-09

- Mas-Colell, Andreu; Whinston, Michael D.; Green, Jerry R. (1995).
*Microeconomic Theory*. New York: Oxford University Press. pp. 9–11. ISBN 0-19-507340-1.

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