A corollary to the Intermediate Value Theorem is that if f(x) is a continuous real-valued function on an interval I, then the set f(I) is also an interval or a single point. Is the converse true? Suppose f(x) is defined on an interval I and that f(I) is an interval. Is f(x) continuous on I? Would the answer change if f(x) is one-to-one?

Jaxon Hamilton

Jaxon Hamilton

Answered question

2022-07-21

A corollary to the Intermediate Value Theorem is that if f ( x ) is a continuous real-valued function on an interval I, then the set f ( I ) is also an interval or a single point.
Is the converse true? Suppose f ( x ) is defined on an interval I and that f ( I ) is an interval. Is f ( x ) continuous on I?
Would the answer change if f ( x ) is one-to-one?

Answer & Explanation

Monica Dennis

Monica Dennis

Beginner2022-07-22Added 13 answers

The conclusion is not true even if f is one to one. Consider the function on [−1,1] defined by f ( x ) = x for x 1 , 0 , 1, and f ( 1 ) = 0 , f ( 0 ) = 1 , f ( 1 ) = 1. Then this function is not continuous, but is one to one and the range f ( I ) is the interval [−1,1].
Haley Madden

Haley Madden

Beginner2022-07-23Added 7 answers

Here is one converse:
If f is monotone and f ( I ) is an interval, then f is continuous.

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