# Undergraduate Mathematics/Intermediate value theorem

In mathematical analysis, the **intermediate value theorem** states that if a continuous function with an interval as its domain takes values and at each end of the interval, then it also takes any value between and at some point within the interval. This has two important specializations: If a continuous function has values of opposite sign inside an interval, then it has a root in that interval (**Bolzano's theorem**). And, the image of a continuous function over an interval is itself an interval.

## Motivation[edit]

This captures an intuitive property of continuous functions: given continuous on with the known values and . Then the graph of must pass through the horizontal line while moves from 1 to 2. It represents the idea that the graph of a continuous function on a closed interval can be drawn without lifting your pencil from the paper.

## Theorem[edit]

The intermediate value theorem states the following:

Consideran interval in the real numbers and a continuous function . Then,

*Version I.*if is a number between and ,

- or ,
- then there is a such that .

*Version II.*the image set is also an interval, and either it contains , or it contains ; that is,

- or .

**Remark:** *Version II* states that the set of function values has no gap. For any two function values , even if they are outside the interval between and , all points in the interval are also function values,

- .

A subset of the real numbers with no internal gap is an interval. *Version I* is obviously contained in *Version II*.

## Relation to Completeness[edit]

The theorem depends on (and is actually equivalent to) the completeness of the real numbers. It is false for the rational numbers . For example, the function for satisfies and . However there is no rational number such that , because is irrational.

## Proof[edit]

The theorem may be proved as a consequence of the completeness property of the real numbers as follows:^{[1]}

We shall prove the first case ; the second is similar.

Let . Then is non-empty since , and is bounded above by .

Hence, by completeness, the supremum exists. That is, is the lowest number that for all . We claim that .

Fix some . Since is continuous, there is a such that whenever .

This means that for all we get

By the properties of the supremum, we have 2 cases:

- There are that are also .

- Hence, there is an that is , such that
- but must imply
- so

- All are definitely .

- Hence, there is an that is , such that
- but must imply
- so

Both inequalities combined

are valid for all , from which we deduce as the only possible value, as stated.

## History[edit]

For above, the statement is also known as *Bolzano's theorem.* This theorem was first proved by Bernard Bolzano in 1817. Augustin-Louis Cauchy provided a proof in 1821.^{[2]} Both were inspired by the goal of formalizing the analysis of functions and the work of Augustin-Louis Cauchy. The idea that continuous functions possess the intermediate value property has an earlier origin. Simon Stevin proved the intermediate value theorem for polynomials (using a cubic as an example) by providing an algorithm for constructing the decimal expansion of the solution. The algorithm iteratively subdivides the interval into 10 parts, producing an additional decimal digit at each step of the iteration.^{[3]} Before the formal definition of continuity was given, the intermediate value property was given as part of the definition of a continuous function. Proponents include Louis Arbogast, who assumed the functions to have no jumps, satisfy the intermediate value property and have increments whose sizes corresponded to the sizes of the increments of the variable.
Earlier authors held the result to be intuitively obvious, and requiring no proof. The insight of Bolzano and Cauchy was to define a general notion of continuity (in terms of Infinitesimals in Cauchy's case, and using real inequalities in Bolzano's case), and to provide a proof based on such definitions.

## Generalization[edit]

The intermediate value theorem can be seen as a consequence of the following two statements from topology:

- If and are topological spaces, is continuous, and is connected, then is connected.
- A subset of is connected if and only if it is an interval.

The intermediate value theorem generalizes in a natural way: Suppose that is a connected topological space and () is a totally ordered set equipped with the order topology, and let be a continuous map. If and are two points in and is a point in lying between and with respect to , then there exists such that . The original theorem is recovered by noting that is connected and that its natural topology is the order topology.

## Converse is false[edit]

A "Darboux function" is a real-valued function that has the "intermediate value property", i.e., that satisfies the conclusion of the intermediate value theorem: for any two values in the domain of , and any between and , there is some with . The intermediate value theorem says that every continuous function is a Darboux function. However, not every Darboux function is continuous; i.e., the converse of the intermediate value theorem is false.

As an example, take the function defined by for and . This function is not continuous at because the limit of as tends to 0 does not exist; yet the function has the intermediate value property. Another, more complicated example is given by the Conway base 13 function.

Historically, this intermediate value property has been suggested as a *definition* for continuity of real-valued functions^{[citation needed]}; this definition was not adopted.

Darboux's theorem states that all functions that result from the differentiation of some other function on some interval have the intermediate value property (even though they need not be continuous).

## Implications of theorem in real world[edit]

The theorem implies that on any great circle around the world, the temperature, pressure, elevation, carbon dioxide concentration, or any other similar scalar quantity which varies continuously, there will always exist two antipodal points that share the same value for that variable.

*Proof:* Take to be any continuous function on a circle. Draw a line through the center of the circle, intersecting it at two opposite points and . Let be defined by the difference . If the line is rotated , the value will be obtained instead. Due to the intermediate value theorem there must be some intermediate rotation angle for which , and as a consequence at this angle.

This is a special case of a more general result called the Borsuk–Ulam theorem.

Another generalization for which this holds is for any closed convex () dimensional shape. Specifically, for any continuous function whose domain is the given shape, and any point inside the shape (not necessarily its center), there exist two antipodal points with respect to the given point whose functional value is the same. The proof is identical to the one given above.

The theorem also underpins the explanation of why rotating a wobbly table will bring it to stability (subject to certain easily-met constraints).^{[4]}

## References[edit]

- ↑ Essentially follows Clarke, Douglas A. (1971).
*Foundations of Analysis*. Appleton-Century-Crofts. p. 284. - ↑ Grabiner, Judith V. (March 1983). "Who Gave You the Epsilon? Cauchy and the Origins of Rigorous Calculus".
*The American Mathematical Monthly*(Mathematical Association of America)**90**(3): 185–194. doi:10.2307/2975545. Archived from the original on 2003-03-30. http://web.archive.org/web/20030330014235/http://www.maa.org/pubs/Calc_articles/ma002.pdf - ↑ Karin Usadi Katz and Mikhail G. Katz (2011) A Burgessian Critique of Nominalistic Tendencies in Contemporary Mathematics and its Historiography. Foundations of Science. doi:10.1007/s10699-011-9223-1 See link
- ↑ Keith Devlin (2007) How to stabilize a wobbly table

## External links[edit]

- Intermediate value Theorem - Bolzano Theorem at cut-the-knot
- Bolzano's Theorem by Julio Cesar de la Yncera, Wolfram Demonstrations Project.