Maxima and minima

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Local and global maxima and minima for cos(3px)/x, 0.1<= x <=1.1 Extrema example original.svg
Local and global maxima and minima for cos(3πx)/x, 0.1 x 1.1

In mathematical analysis, the maxima and minima (the respective plurals of maximum and minimum) of a function, known collectively as extrema (the plural of extremum), are the largest and smallest value of the function, either within a given range (the local or relative extrema), or on the entire domain (the global or absolute extrema). [1] [2] [3] Pierre de Fermat was one of the first mathematicians to propose a general technique, adequality, for finding the maxima and minima of functions.

Contents

As defined in set theory, the maximum and minimum of a set are the greatest and least elements in the set, respectively. Unbounded infinite sets, such as the set of real numbers, have no minimum or maximum.

Definition

A real-valued function f defined on a domain X has a global (or absolute) maximum point at x, if f(x) ≥ f(x) for all x in X. Similarly, the function has a global (or absolute) minimum point at x, if f(x) ≤ f(x) for all x in X. The value of the function at a maximum point is called the maximum value of the function, denoted , [4] and the value of the function at a minimum point is called the minimum value of the function. Symbolically, this can be written as follows:

is a global maximum point of function if

The definition of global minimum point also proceeds similarly.

If the domain X is a metric space, then f is said to have a local (or relative) maximum point at the point x, if there exists some ε > 0 such that f(x) ≥ f(x) for all x in X within distance ε of x. Similarly, the function has a local minimum point at x, if f(x) ≤ f(x) for all x in X within distance ε of x. A similar definition can be used when X is a topological space, since the definition just given can be rephrased in terms of neighbourhoods. Mathematically, the given definition is written as follows:

Let be a metric space and function . Then is a local maximum point of function if such that

The definition of local minimum point can also proceed similarly.

In both the global and local cases, the concept of a strict extremum can be defined. For example, x is a strict global maximum point if for all x in X with xx, we have f(x) > f(x), and x is a strict local maximum point if there exists some ε > 0 such that, for all x in X within distance ε of x with xx, we have f(x) > f(x). Note that a point is a strict global maximum point if and only if it is the unique global maximum point, and similarly for minimum points.

A continuous real-valued function with a compact domain always has a maximum point and a minimum point. An important example is a function whose domain is a closed and bounded interval of real numbers (see the graph above).

Finding global maxima and minima is the goal of mathematical optimization. If a function is continuous on a closed interval, then by the extreme value theorem, global maxima and minima exist. Furthermore, a global maximum (or minimum) either must be a local maximum (or minimum) in the interior of the domain, or must lie on the boundary of the domain. So a method of finding a global maximum (or minimum) is to look at all the local maxima (or minima) in the interior, and also look at the maxima (or minima) of the points on the boundary, and take the largest (or smallest) one.

Likely the most important, yet quite obvious, feature of continuous real-valued functions of a real variable is that they decrease before local minima and increase afterwards, likewise for maxima. (Formally, if f is continuous real-valued function of a real variable x, then x0 is a local minimum if and only if there exist a < x0 < b such that f decreases on (a, x0) and increases on (x0, b)) [5] A direct consequence of this is the Fermat's theorem, which states that local extrema must occur at critical points (or points where the function is non-differentiable). [6] One can distinguish whether a critical point is a local maximum or local minimum by using the first derivative test, second derivative test, or higher-order derivative test, given sufficient differentiability. [7]

For any function that is defined piecewise, one finds a maximum (or minimum) by finding the maximum (or minimum) of each piece separately, and then seeing which one is largest (or smallest).

Examples

The global maximum of [?]x occurs at x = e. Xth root of x.svg
The global maximum of x occurs at x = e .
FunctionMaxima and minima
x2Unique global minimum at x = 0.
x3No global minima or maxima. Although the first derivative (3x2) is 0 at x = 0, this is an inflection point. (2nd derivative is 0 at that point.)
Unique global maximum at x = e . (See figure at right)
xxUnique global maximum over the positive real numbers at x = 1/e.
x3/3 − xFirst derivative x2 − 1 and second derivative 2x. Setting the first derivative to 0 and solving for x gives stationary points at −1 and +1. From the sign of the second derivative, we can see that −1 is a local maximum and +1 is a local minimum. This function has no global maximum or minimum.
|x|Global minimum at x = 0 that cannot be found by taking derivatives, because the derivative does not exist at x = 0.
cos(x)Infinitely many global maxima at 0, ±2π, ±4π, ..., and infinitely many global minima at ±π, ±3π, ±5π, ....
2 cos(x) − xInfinitely many local maxima and minima, but no global maximum or minimum.
cos(3πx)/x with 0.1 ≤ x ≤ 1.1Global maximum at x = 0.1 (a boundary), a global minimum near x = 0.3, a local maximum near x = 0.6, and a local minimum near x = 1.0. (See figure at top of page.)
x3 + 3x2 − 2x + 1 defined over the closed interval (segment) [−4,2]Local maximum at x = −1−15/3, local minimum at x = −1+15/3, global maximum at x = 2 and global minimum at x = −4.

For a practical example, [8] assume a situation where someone has feet of fencing and is trying to maximize the square footage of a rectangular enclosure, where is the length, is the width, and is the area:

The derivative with respect to is:

Setting this equal to

reveals that is our only critical point. Now retrieve the endpoints by determining the interval to which is restricted. Since width is positive, then , and since , that implies that . Plug in critical point , as well as endpoints and , into , and the results are and respectively.

Therefore, the greatest area attainable with a rectangle of feet of fencing is . [8]

Functions of more than one variable

Peano surface, a counterexample to some criteria of local maxima of the 19th century Modell einer Peanoschen Flache -Schilling XLIX, 1-.jpg
Peano surface, a counterexample to some criteria of local maxima of the 19th century
The global maximum is the point at the top MaximumParaboloid.png
The global maximum is the point at the top
Counterexample: The red dot shows a local minimum that is not a global minimum MaximumCounterexample.png
Counterexample: The red dot shows a local minimum that is not a global minimum

For functions of more than one variable, similar conditions apply. For example, in the (enlargeable) figure on the right, the necessary conditions for a local maximum are similar to those of a function with only one variable. The first partial derivatives as to z (the variable to be maximized) are zero at the maximum (the glowing dot on top in the figure). The second partial derivatives are negative. These are only necessary, not sufficient, conditions for a local maximum, because of the possibility of a saddle point. For use of these conditions to solve for a maximum, the function z must also be differentiable throughout. The second partial derivative test can help classify the point as a relative maximum or relative minimum. In contrast, there are substantial differences between functions of one variable and functions of more than one variable in the identification of global extrema. For example, if a bounded differentiable function f defined on a closed interval in the real line has a single critical point, which is a local minimum, then it is also a global minimum (use the intermediate value theorem and Rolle's theorem to prove this by reductio ad impossibile). In two and more dimensions, this argument fails. This is illustrated by the function

whose only critical point is at (0,0), which is a local minimum with f(0,0) = 0. However, it cannot be a global one, because f(2,3) = −5.

Maxima or minima of a functional

If the domain of a function for which an extremum is to be found consists itself of functions (i.e. if an extremum is to be found of a functional), then the extremum is found using the calculus of variations.

In relation to sets

Maxima and minima can also be defined for sets. In general, if an ordered set S has a greatest element m, then m is a maximal element of the set, also denoted as . [4] Furthermore, if S is a subset of an ordered set T and m is the greatest element of S with (respect to order induced by T), then m is a least upper bound of S in T. Similar results hold for least element, minimal element and greatest lower bound. The maximum and minimum function for sets are used in databases, and can be computed rapidly, since the maximum (or minimum) of a set can be computed from the maxima of a partition; formally, they are self-decomposable aggregation functions.

In the case of a general partial order, the least element (i.e., one that is smaller than all others) should not be confused with a minimal element (nothing is smaller). Likewise, a greatest element of a partially ordered set (poset) is an upper bound of the set which is contained within the set, whereas a maximal elementm of a poset A is an element of A such that if mb (for any b in A), then m = b. Any least element or greatest element of a poset is unique, but a poset can have several minimal or maximal elements. If a poset has more than one maximal element, then these elements will not be mutually comparable.

In a totally ordered set, or chain, all elements are mutually comparable, so such a set can have at most one minimal element and at most one maximal element. Then, due to mutual comparability, the minimal element will also be the least element, and the maximal element will also be the greatest element. Thus in a totally ordered set, we can simply use the terms minimum and maximum.

If a chain is finite, then it will always have a maximum and a minimum. If a chain is infinite, then it need not have a maximum or a minimum. For example, the set of natural numbers has no maximum, though it has a minimum. If an infinite chain S is bounded, then the closure Cl(S) of the set occasionally has a minimum and a maximum, in which case they are called the greatest lower bound and the least upper bound of the set S, respectively.

See also

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In mathematical optimization, the method of Lagrange multipliers is a strategy for finding the local maxima and minima of a function subject to equality constraints. It is named after the mathematician Joseph-Louis Lagrange. The basic idea is to convert a constrained problem into a form such that the derivative test of an unconstrained problem can still be applied. The relationship between the gradient of the function and gradients of the constraints rather naturally leads to a reformulation of the original problem, known as the Lagrangian function.

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Maximal and minimal elements An element of a preordered set that is the only element ≥ (respectively, ≤) to itself (in contrast, greatest elements are ≥ all elements).

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Saddle point

In mathematics, a saddle point or minimax point is a point on the surface of the graph of a function where the slopes (derivatives) in orthogonal directions are all zero, but which is not a local extremum of the function. An example of a saddle point is when there is a critical point with a relative minimum along one axial direction and at a relative maximum along the crossing axis. However, a saddle point need not be in this form. For example, the function has a critical point at that is a saddle point since it is neither a relative maximum nor relative minimum, but it does not have a relative maximum or relative minimum in the -direction.

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Stationary point

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Most of the terms listed in Wikipedia glossaries are already defined and explained within Wikipedia itself. However, glossaries like this one are useful for looking up, comparing and reviewing large numbers of terms together. You can help enhance this page by adding new terms or writing definitions for existing ones.

References

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