In mathematics a constant function is a function whose values do not vary and thus are constant. For example, if we have the function f(x) = 4, then f is constant since f maps any value to 4. More formally, a function f : A → B, is a constant function if f(x) = f(y) for all x and y in A.
Notice that every empty function, that is, any function whose domain equals the empty set, is included in the above definition vacuously, since there are no x and y in A for which f(x) and f(y) are different. However some find it more convenient to define constant function so as to exclude empty functions.
For polynomial functions, a non-zero constant function is called a polynomial of degree zero.
Properties
Constant functions can be characterized with respect to function composition in two ways.
The following are equivalent:
- f : A → B, is a constant function.
- For all functions g, h : C → A, f o g = f o h, (where “o” denotes function composition).
- The composition of f with any other function is also a constant function.
The first characterization of constant functions given above, is taken as the motivating and defining property for the more general notion of constant morphism in Category theory.
In contexts where it is defined, the derivative of a function measures how that function varies with respect to the variation of some argument. It follows that, since a constant function does not vary, its derivative, where defined, will be zero. Thus for example:
- If f is a real-valued function of a real variable, defined on some interval, then f is constant if and only if the derivative of f is everywhere zero.
For functions between preordered sets, constant functions are both order-preserving and order-reversing; conversely, if f is both order-preserving and order-reversing, and if the domain of f is a lattice, then f must be constant.
Other properties of constant functions include:
- Every constant function whose domain and codomain are the same is idempotent.
- Every constant function between topological spaces is continuous.
In a connected set, a function is locally constant if and only if it is constant.
References
- Herrlich, Horst and Strecker, George E., Category Theory, Allen and Bacon, Inc. Boston (1973)
-
Saturday, January 12th, 2008
In economics, a consumer’s indirect utility function
<math>v(p, w)</math> gives the consumer’s maximal utility when faced with a price level <math>p</math> and an amount of income <math>w</math>. It represents the consumer’s preferences over market conditions.
This function is called indirect because consumers usually think about their preferences in terms of what they consume rather than prices. A consumer’s indirect utility <math>v(p, w)</math> can be computed from its utility function <math>u(x)</math> by first computing the most preferred bundle <math>x(p, w)</math> by solving the utility maximization problem; and second, computing the utility <math>u(x(p, w))</math> the consumer derives from that bundle. The indirect utility function for consumers is analogous to the profit function for firms.
Formally, the indirect utility function is:
- Non-increasing in prices, because an increase in prices cannot open up an available bundle that would provide more utility;
- Non-decreasing in income, because when income rises, at worst you could consume the same bundle;
- Homogenous with degree zero in prices and income; if prices and income are all multiplied by a given constant the same bundle of consumption represents a maximum, so optimal utility does not change.
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Thursday, January 10th, 2008
In microeconomics, the expenditure function describes the minimum amount of money an individual needs to achieve some level of utility, given a utility function and prices.
Formally, if there is a utility function <math>u</math> that describes preferences over L commodities, the expenditure function
- <math>e(p, u^*) : \textbf R^L_+ \times \textbf R
\rightarrow \textbf R</math>
says what amount of money is needed to achieve a utility <math>u^*</math> if prices are set by <math>p</math>.
This function is defined by
- <math>e(p, u^*) = \min_{x \in \geq(u^*)} p \cdot x</math>
where
- <math>\geq(u^*) = \{x \in \textbf R^L_+ : u(x) \geq u^*\}</math>
is the set of all packages that give utility at least as good as <math>u^*</math>.
See also
- Expenditure minimization problem
- Hicksian demand function
- Utility maximization problem
Tuesday, January 8th, 2008
In mathematics, a locally integrable function is a function which is integrable on any compact set.
Formally, let U be an open set in the Euclidean space Rn and
- <math>f\colon U\to\mathbb{C}</math>
be a Lebesgue measurable function. If the Lebesgue integral
- <math> \int_K | f| dx \,</math>
is finite for all compact subsets K in U, then f is called locally integrable. The set of all such functions is denoted by
- <math>L^1_{loc}(U).</math>
Examples
- Every (globally) integrable function on U is locally integrable, that is,
-
- <math>L^1(U)\subset L^1_{loc}(U)</math> (see Lp space).
- More generally, every p-power integrable function (1 ≤ p ≤ ∞) on U is locally integrable:
-
- <math>L^p(U)\subset L^1_{loc}(U)</math>.
- The constant function 1 defined on the real line is locally integrable but not globally integrable. More generally, continuous functions are locally integrable.
- The function <math>f(x)=1/x</math> for <math>x\neq 0</math> and <math>f(0)=0</math> is not locally integrable.
Uses
Locally integrable functions play a prominent role in distribution theory.
References
- Robert S Strichartz, A Guide to Distribution Theory and Fourier Transforms, World Scientific, 2003. ISBN 981-238-430-8.
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Monday, December 31st, 2007
In microeconomics, the expenditure function describes the minimum amount of money an individual needs to achieve some level of utility, given a utility function and prices.
Formally, if there is a utility function <math>u</math> that describes preferences over L commodities, the expenditure function
- <math>e(p, u^*) : \textbf R^L_+ \times \textbf R
\rightarrow \textbf R</math>
says what amount of money is needed to achieve a utility <math>u^*</math> if prices are set by <math>p</math>.
This function is defined by
- <math>e(p, u^*) = \min_{x \in \geq(u^*)} p \cdot x</math>
where
- <math>\geq(u^*) = \{x \in \textbf R^L_+ : u(x) \geq u^*\}</math>
is the set of all packages that give utility at least as good as <math>u^*</math>.
See also
- Expenditure minimization problem
- Hicksian demand function
- Utility maximization problem
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Sunday, December 30th, 2007
In microeconomics, a consumer’s Hicksian demand function <math>h(p, u)</math> gives the cheapest bundle under a price level <math>p</math> for which the consumer derives a utility level of at least <math>u</math>. The function is named after John Hicks.
Hicksian demand functions are often convenient for mathematical manipulation because they don’t require income or wealth to be represented. However, Marshallian demand functions of the form <math>x(p, w)</math> that describe demand given prices <math>p</math> and income <math>w</math> are easier to observe directly. The two are trivially related by
- <math>h(p, u) = x(p, e(p, u)), \ </math>
where <math>e(p, u)</math> is the expenditure function (the function that gives the minimum wealth required to get to a given utility level), and by
- <math>h(p, v(p, w)) = x(p, w), \ </math>
where <math>v(p, w)</math> is the indirect utility function (which gives the utility level of having a given wealth under a fixed price regime). Their derivatives are more fundamentally related by the Slutsky equation.
The Hicksian demand function is intimately related to the expenditure function. If the consumer’s utility function <math>u(x)</math> is locally nonsatiated and strictly convex, then
<math>h(p, u) = \nabla_p e(p, u).</math>
See also
- Marshallian demand function
- Convex preferences
- Expenditure minimization problem
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Monday, December 24th, 2007
Bidirectional texture function (BTF) is a 6-dimensional function depending on planar texture coordinates (x,y) as well as on view and illumination spherical angles. In practice this function is obtained as a set of several thousands images of material sample taken during different camera and light positions. Such images are subsequently rectified and compressed using various methods.
Its main application is photorealistic material rendering of objects in virtual reality systems.
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Thursday, December 20th, 2007
In mathematics, a function f(x) between two ordered sets is unimodal if for some value m (the mode), it is monotonically increasing for x ≤ m and monotonically decreasing for x ≥ m. In that case, the maximum value of f(x) is f(m) and there are no other local maxima.
Examples of unimodal function:
- Quadratic polynomial
- Logistic map
- Tent map
Function <math>\ f(x)</math> is S-unimodal if its Schwartzian derivative is negative for all <math>\ x \ne 0</math> <ref>http://web.udl.es/usuaris/y4370980/abstracts/abstracts/vol-42/de_melo.pdf W. De Melo : Bifurcation of Unimodal Maps Qualitative Theory of Dynamical Systems VOLUME 4 - Number 2 (pages 413-424) </ref>.
In probability and statistics, a unimodal probability distribution is a probability distribution whose probability density function is a unimodal function, or more generally, whose cumulative distribution function is convex up to m and concave thereafter (this allows for the possibility of a non-zero probability for x=m). For a unimodal probability distribution of a continuous random variable, the Vysochanskii-Petunin inequality provides a refinement of the Chebyshev inequality. Compare multimodal distribution.
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Thursday, December 20th, 2007
In mathematics, characteristic function can refer to any of several distinct concepts:
- The most common and universal usage is as a synonym for indicator function, that is the function
-
- <math>\mathbf{1}_A: X \to \{0, 1\}</math>
- which for every subset A of X, has value 1 at points of A and 0 at points of X − A.
- When applied to a natural number an effective procedure determines correctly if a natural number is or is not in the procedure’s “set”: “The characteristic function is the function that takes the value 1 for numbers in the set, and the value 0 for numbers not in the set” (cf Boolos-Burgess-Jeffrey (2002) p. 73).
- The characteristic function in convex analysis:
-
- <math>\chi_{A} (x) := \begin{cases} 0, & x \in A; \\ + \infty, & x \not \in A. \end{cases}</math>
- The characteristic state function in statistical mechanics.
- In probability theory, the characteristic function of any probability distribution on the real line is given by the following formula, where X is any random variable with the distribution in question:
-
- <math>\varphi_X(t) = \operatorname{E}\left(e^{itX}\right)\,</math>
- where “E” means expected value. See characteristic function (probability theory).
- The characteristic polynomial in linear algebra.
- The Euler characteristic, a topological invariant.
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Monday, December 17th, 2007
In mathematics, by sigma function one can mean one of the following:
- The sum-of-divisors function σa(n), an arithmetic functions.
- Weierstrass sigma function, related to elliptic functions
- Rado’s sigma function, see busy beaver.
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Sunday, December 9th, 2007
In mathematics, by sigma function one can mean one of the following:
- The sum-of-divisors function σa(n), an arithmetic functions.
- Weierstrass sigma function, related to elliptic functions
- Rado’s sigma function, see busy beaver.
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Sunday, December 9th, 2007
In mathematics, the psi function can refer to either
- Dedekind psi function
- Digamma function
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Thursday, December 6th, 2007
A single-valued function is an emphatic term for a mathematical function in the usual sense. That is, each element of the function’s domain maps to a single, well-defined element of its range. This contrasts with a general binary relation, which can be viewed as being a multi-valued function.
Any function is a single-valued if it is continuously differentiable.
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Tuesday, December 4th, 2007
In mathematics, the q-theta function is a type of q-series. It is given by
- <math>\theta(z;q)=\prod_{n=0}^\infty (1-q^nz)(1-q^{n+1}/z)</math>
where one takes <math>0\le|q|<1.</math> It obeys the identities
- <math>\theta(z;q)=\theta(q/z;q)=-z\theta(1/z;q).\,</math>
See also
- Jacobi theta function
- Ramanujan theta function