Proving One to One Functions: Understanding Strictly Increasing Intervals

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In summary, a function from the real numbers to the real numbers is one-to-one on an interval I if it is strictly increasing on that interval. To prove this, one can use a contradiction by assuming that there are distinct points in the interval with equal function values. The use of the strictly increasing property will lead to a contradiction, proving that the function is indeed one-to-one.
  • #1
staw_jo
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Homework Statement



A function from the real numbers to the real numbers is one to one on an interval I if it is strictly increasing on that interval.

Any help please!


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The Attempt at a Solution



I am not quite sure how to prove it, I know that the use of strictly increasing is important as far as if x1 < x2, then f(x1) < f(x2). A hint I was told to use is contradiction.
 
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  • #2
If you would like to prove it by contradiction, assume there is such a time that f(x) = f(y) where x is not y. Then either x < y or y < x. Now use the strictly increasing property of the function.
 
  • #3
To prove it by contradiction, negate the definition of one-to-one. Suppose that there are distinct points, a and b, in I, such that f(a) = f(b). You know that either a < b or b < a, right?
 
  • #4
Great :smile:
 
  • #5
Okay so:

Assume that f(x1) = f(x2), but x1 does not equal x2, then either x1 < x2 or x2 < x1, since it is strictly increasing, this implies that f(x1) < f(x2) or f(x2) < f(x1), so f(x1) can never equal f(x2), therefore the function must be one to one.

Is this what you are saying?
 
  • #6
Yes, that is the right idea.

For clarity, instead of saying "f(x1) can never equal f(x2)", just state that "f(x1) < f(x2) or f(x2) < f(x1)" is a contradiction with the fact that f(x1) = f(x2) and thus the function is one-to-one.
 
  • #7
Alright, thank you SO much for your help!
 
  • #8
Great, glad I could help.
 

FAQ: Proving One to One Functions: Understanding Strictly Increasing Intervals

What is a one-to-one function?

A one-to-one function is a type of mathematical function in which each input (x-value) has a unique output (y-value). This means that no two different inputs can produce the same output. It is also known as an injective function.

How do you prove that a function is one-to-one?

To prove that a function is one-to-one, you must show that for any two distinct inputs, the corresponding outputs are also distinct. This can be done algebraically by setting the two inputs equal to each other and solving for the variable. If the outputs are not equal, then the function is one-to-one.

What is a strictly increasing interval?

A strictly increasing interval is a range of values for which the function is increasing, meaning that the output values are getting larger as the input values increase. In other words, the slope of the function is positive within this interval.

How do you determine the strictly increasing intervals of a function?

To determine the strictly increasing intervals of a function, you must find the intervals where the function's derivative (slope) is positive. This can be done by finding the critical points (where the derivative is equal to zero or undefined) and testing points on either side of these points to see if the derivative is positive.

Can a function be both one-to-one and strictly increasing?

Yes, a function can be both one-to-one and strictly increasing. In fact, all one-to-one functions are also strictly increasing, as each input has a unique output. However, not all strictly increasing functions are one-to-one, as there may be different inputs that produce the same output.

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