Intermediate value theorem
In mathematical analysis, the intermediate value theorem states that for each value between the least upper bound and greatest lower bound of the image of a continuous function there is at least one point in its domain that the function maps to that value.
- Version I. The intermediate value theorem states the following: If f is a real-valued continuous function on the interval [a, b], and u is a number between f(a) and f(b), then there is a c ∈ [a, b] such that f(c) = u.
- Version II. Suppose that I is an interval [a, b] in the real numbers R and that f : I → R is a continuous function. Then the image set f(I) is also an interval, and either it contains [f(a), f(b)], or it contains [f(b), f(a)]; that is,
- f(I) ⊇ [f(a), f(b)], or f(I) ⊇ [f(b), f(a)].
It is frequently stated in the following equivalent form: Suppose that f : [a, b] → R is continuous and that u is a real number satisfying f(a) < u < f(b) or f(a) > u > f(b). Then for some c ∈ [a, b], f(c) = u.
This captures an intuitive property of continuous functions: given f continuous on [1, 2], if f(1) = 3 and f(2) = 5 then f must take the value 4 somewhere between 1 and 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.
The theorem depends on (and is actually equivalent to) the completeness of the real numbers. It is false for the rational numbers Q. For example, the function f(x) = x2 − 2 for x ∈ Q satisfies f(0) = −2 and f(2) = 2. However there is no rational number x such that f(x) = 0, because √2 is irrational.
We shall prove the first case f(a) < u < f(b); the second is similar.
Let S be the set of all x in [a, b] such that f(x) ≤ u. Then S is non-empty since a is an element of S, and S is bounded above by b. Hence, by completeness, the supremum c = sup S exists. That is, c is the lowest number that is greater than or equal to every member of S. We claim that f(c) = u.
- Suppose first that f(c) > u, then f(c) − u > 0. Since f is continuous, there is a δ > 0 such that | f(x) − f(c) | < ε whenever | x − c | < δ. Pick ε = f(c) − u, then | f(x) − f(c) | < f(c) − u. But then, f(x) > f(c) − (f(c) − u) = u whenever | x − c | < δ (that is, f(x) > u for x in (c − δ, c + δ)). This requires that c − δ be an upper bound for S (since no point in the interval (c − δ, c] for which f > u, can be contained in S, and c was defined as the least upper bound for S), an upper bound less than c. The contradiction nullifies this paragraph's opening assumption.
- Suppose instead that f(c) < u. Again, by continuity, there is a δ > 0 such that | f(x) − f(c) | < u − f(c) whenever | x − c | < δ. Then f(x) < f(c) + (u − f(c)) = u for x in (c − δ, c + δ). Since x=c + δ/2 is contained in (c − δ, c + δ), it also satisfies f(x) < u, so it must be contained in S. However, it also exceeds the least upper bound c of S. The contradiction nullifies this paragraph's opening assumption, as well.
We deduce that f(c) = u as stated.
An alternative proof may be found at non-standard calculus.
For u = 0 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. Both were inspired by the goal of formalizing the analysis of functions and the work of Joseph-Louis Lagrange. 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. 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.
The intermediate value theorem can be seen as a consequence of the following two statements from topology:
- If X and Y are topological spaces, f : X → Y is continuous, and X is connected, then f(X) is connected.
- A subset of R is connected if and only if it is an interval.
The intermediate value theorem generalizes in a natural way: Suppose that X is a connected topological space and (Y, <) is a totally ordered set equipped with the order topology, and let f : X → Y be a continuous map. If a and b are two points in X and u is a point in Y lying between f(a) and f(b) with respect to <, then there exists c in X such that f(c) = u. The original theorem is recovered by noting that R is connected and that its natural topology is the order topology.
Intermediate value theorem and the completeness axiom 
As we showed above, the Intermediate value theorem can be proved using the completeness axiom. In fact, the intermediate value theorem is equivalent to the completeness axiom; that is to say, any unbounded dense subset S of R to which the intermediate value theorem applies must also satisfy the completeness axiom.
To show this, take some bounded-above subset A of S. We will show that A has a least upper bound, using the intermediate value theorem. Consider the function f : S → [0, 1] defined by f(x) = 1 if x is an upper bound for A (i.e., for all s ∈ A, x ≥ s) and 0 otherwise. If A has only one element then that element is a least upper bound for A. Otherwise, there is some element a in A, and hence in S, which is not an upper bound for A. So f(a) = 0. Because A is bounded above and S is bounded, there exists b ∈ S such that b is an upper bound for S. So f(b) = 1. By the intermediate value theorem, if f is continuous then there must be some y ∈ S such that f(y) = ½. Since there is no such y, f is not continuous. So there is some point z in S such that for some ε > 0, for all δ > 0, f is not continuous at z.
f(z) cannot equal 0, though: if f(z) = 0 then z is not an upper bound for A, so any w < z is not an upper bound for A. So since f is discontinuous at z, there exists some point p in the range [z, z + δ], for any δ > 0, such that p is an upper bound for A. But then this contradicts z not being an upper bound for A: suppose m ∈ A such that m > z. We know that there exists p with z < p < z + ½ ( m - z ) which is an upper bound for A (setting δ = ½ ( m - z ) ), so p > m. But p < z + ½ ( m - z ) < m, which is a contradiction.
So f(z) = 1, so z is an upper bound for A. Since for all v > z, v is an upper bound for A, f(v) = 1 for all v > z. Since f is not continuous at z, there there exists some point q in the range [z - δ, z], for any δ > 0, such that q is not an upper bound for A. Now suppose there is some l ∈ S such that l < z and l is an upper bound for A. We then set δ to be ½( l - z ), so there is some r ∈ [z - ½ ( l - z ), z] which is not an upper bound for A. But then l < z - ½ ( l - z ) < r, so l is not an upper bound for A, which is a contradiction.
So z is a least upper bound for A.
Converse is false 
A "Darboux function" is a real-valued function f that has the "intermediate value property", i.e., that satisfies the conclusion of the intermediate value theorem: for any two values a and b in the domain of f, and any y between f(a) and f(b), there is some c between a and b with f(c) = y. 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 f : [0, ∞) → [−1, 1] defined by f(x) = sin(1/x) for x > 0 and f(0) = 0. This function is not continuous at x = 0 because the limit of f(x) as x 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; 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 
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 f to be any continuous function on a circle. Draw a line through the center of the circle, intersecting it at two opposite points A and B. Let d be defined by the difference f(A) − f(B). If the line is rotated 180 degrees, the value −d will be obtained instead. Due to the intermediate value theorem there must be some intermediate rotation angle for which d = 0, and as a consequence f(A) = f(B) 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 n (n>1) 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).
See also 
- 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. JSTOR 2975545 More than one of
- 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
- O'Connor, John J.; Robertson, Edmund F., "Intermediate value theorem", MacTutor History of Mathematics archive, University of St Andrews.
- Keith Devlin (2007) How to stabilize a wobbly table