intermediate value theorem

In calculus, the intermediate value theorem is either of two theorems of which an account is given below.

Intermediate value theorem

The intermediate value theorem states the following: Suppose that I is an interval in the real numbers R and that f : I -> R is a continuous function. Then the image set f ( I ) is also an interval. It is frequently stated in the following equivalent form: Suppose that f : 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 in (a, b), f(c) = u. This captures an intuitive property of continuous functions: if f (1) = 3 and f (2) = 5 then f must be equal to 4 somewhere between 1 and 2. It represents the idea that the graph of a continuous function can be drawn without lifting your pencil from the paper.

Proof

We shall prove the first case f (a) < u < f (b); the second is similar. Let S = {x in b : f(x) ≤ u}. Then S is non-empty (as a is in S) and bounded above by b. Hence by the completeness property of the real numbers, the supremum c = sup S exists. We claim that f (c) = u. Suppose first that f (c) > u. Then f (c) - u > 0, so there is a δ > 0 such that | f (x) - f (c) | < f (c) - u whenever | x - c | < δ, since f is continuous. But then f (x) > f (c) - ( f (c) - u ) = u whenever | x - c | < δ and then f (x) > u for x in ( c - δ, c + δ) and thus c - δ is an upper bound for S which is smaller than c, a contradiction. Suppose next that f (c) < u. Again, by continuity, there is an 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) and there are numbers x greater than c for which f (x) < u, again a contradiction to the definition of c. We deduce that f (c) = u as stated.

History

For u=0 above, the statement is also known as Bolzano's theorem; this theorem was first stated, together with a proof which used techniques which are now regarded as non-rigorous, by Bernard Bolzano.

Generalization

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.

Converse is false

Suppose f is a real-valued function defined on some interval I, and for every two elements a and b in I and for every u between f(a) and f(b) there exists a c between a and b such that f(c) = u. Does have f have to be continuous? The answer is no; the converse of the intermediate value theorem fails. As an example, take the function f(x) = sin(1/x) for x non-zero, and f(0) = 0. This function is not continuous as the limit for x → 0 does not exist; yet the function has the above intermediate value property. 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).