 INVERSE FUNCTIONS Section: Analysis

1. DEFINING A FUNCTION

Given that we are going to talk about inverse functions (sometimes called reciprocals), we should first make sure that the concept of a function is clear. Although we have already worked with functions several times in both Maths and Physics, have we asked ourselves at any point what a function actually is? Could you give an accurate definition of a function? We all have an intuitive idea of what a function is, but we need to try and express it in mathematical terms. It may seem as if we are leaving our intuition aside by thinking of a formal definition. However, we should try to see how our intuition links to the formal definition, which has already been expressed in straightforward language by those who thought out and/or sensed the definition intuitively.

Throughout history we have seen several definitions of a function. In this unit we are going to refer to the most widely used definition in the field of Modern Mathematics.

Before giving the definition of the function we should remember that:

• RxR = R2, Cartesian product of R by R, is the set of all ordered pairs (x, y), where both x and y are real numbers. In other words:
 RxR = R2 = { (x, y) / x, y Î R }
• An ordered pair of numbers can be represented on the Coordinate System (two straight lines which cross each other at a right angle). We call the first element in a pair of ordered numbers the first coordinate or x-coordinate and the second is the second coordinate or y-coordinate.
• A set can be defined by giving a list of all its members (listing all the elements individually) or by describing a certain property which needs to be satisfied (defining through understanding).
 DEFINITION: A function f is a subset of RxR such that no two different pairs of f have the same first coordinate. In other words, if two pairs of f have the same first member, then the second member is also the same; thus, if (a, b), (a, c) Î f, then b=c.

EXAMPLES:

• f = { (1, 2), (2, 4), (3, -1), (4, 2) } is a function.
• g = { (1, 2), (2, 1), (1, 3), (3, -1) } is not a function as the pairs of numbers (1, 2) and (1, 3) have the same first coordinate and according to the definition 2 is equal to 3 (2=3), which is not true.

When a set is written out as a list of pairs of numbers it is easy to see whether we are dealing with a function or not. However, if the function is defined by a property it can be very difficult to determine whether it is a function or not. The ability to distinguish will depend on your mathematical knowledge.

EXAMPLES:

1. f = { (x, y) / y=2x } is a function, as the value of y can only be determined from x.
2. g = { (a, b) / a2+b2=9 } is not a function as the pairs of numbers (0, 3) and (0, -3) have the same first coordinate and a different second coordinate.
3. h = { (a, b) / a2+b3=16 } is a function; by getting b on its own we can see that each value of a determines one and only one value of b.
4. k = { (x, y) / x3+y3=16xy } is not a function; It is not easy to show that this is the case.

In the following window we can see the graph of each of the four examples above. Use the vertical straight line joined to the control point (in red) to determine which are graphs of functions and which are not.

 In graphical terms the definition of a function is a curve where no two points of the function are found on the same vertical straight line.

1.- Use the window to prove these statements:

1. f = { (x, y) / y=2x } is a function.
2. g = { (a, b) / a2+b2=9 } is not a function.
3. h = { (a, b) / a2+b3=16 } is a function.
4. k = { (x, y) / x3+y3=16xy } is not a function.
 The Nippe Descartes program will give the coordinates of any point, apart from those which coincide with the "control" point, by clicking on this point with the main mouse button.

2.- Use the window to find three points belonging to the set in example 4 whose first coordinate is 8. The results you obtain will only be approximate. Use a calculator to check how good these results are.

When a function is defined by a property it is not necessarily defined by a single formula. It could be as complex as the following example, or perhaps even more so:

f0 = { (x, y) / x2+y2=9 if y>=0; x2+y3=16 if y<0}

3.- Look carefully at graphs 2 and 3 in the first window to help you understand why this is the graph of the function f0.

2. THE DOMAIN AND RANGE OF A FUNCTION
 DEFINITION: If f is a function then the domain of the function is the set of the first coordinates in the pairs of f and the range is the set of the second coordinates in the pairs of f. If the pair (a, b) belongs to f then we call b the image of a, which can be written f(a)=b. Note that in order to define a function there is only one b for each a in the domain.

1.- Study the following examples which show how to calculate the domain and range:

• Let the function f = { (1, 2), (2, 4), (3, -1), (4, 2) }. Its domain is the set { 1, 2, 3, 4}. Its range is { 2, 4, -1 }. f(1)=2, f(2)=4, f(3)=-1 and f(4)=2.
• Let the function h = { (a, b) / a2+b3=16 } = { (x, y) / y=(16-x2)1/3 }. By looking carefully at its graph in the first window we can deduce that its domain is all of R; its range is the set of numbers less than or equal to 161/3 (approximately 2.52); and, for example, h(-4)=0, h(0)=161/3.
 Now it is time to bring our idea of a function closer to that stated by Modern Mathematics. When we said: let the function y=f(x)=x2 whose domain is the interval [-2, 4], what we were saying was that each number of the interval [-2, 4] is mapped to its square and everything else is ignored. In terms of sets it would be expressed as follows: f = { (x, y) / y=x2 if -2 <= x <= 4 } = { (x, x2) / -2 <= x <= 4 }. Most of the time we do not even state the domain of the function. We simply say for example: let the function y = f(x) = (1-3x)/(4-x2), taking for granted that the domain of the function is the largest set where the operations in the expression are possible. This way of defining a function will naturally continue to be used. However, we should not forget that a function is a set of ordered pairs and the equation of the function tells us how the two numbers in the pair are related together. The most important thing about a function f is that only one number f(x) is determined for each value of x in its domain. If we consider all the definitions we have seen so far, we could say that a function f is the set: f = { (x, y) / y=f(x), for all values of x in the domain of f } = { (x, f(x)) / x in the domain of f }   After all of this, our "old" idea of a function should coincide with this "new" idea. The modern definition of a function may seem complicated and may not appear to take us anywhere, but understanding this definition is the best way of going on to understand inverse functions.

2.- Find the domain and range of the function f0 represented in the second window. It is worth looking back at the window to help you find the correct answer. What is the value of f0(3), f0(0), f0(-1), f0(-4), f0(8) and f0(3.5)?

We already know that a function is a set of pairs of numbers. If we reverse the numbers in the pairs we get a new function. Let's do so with the following function:

f = { (1, 2), (2, 4), (3, -1), (4, -2) }

We can see what happens and call this new set g:

g = { (2, 1), (4, 2), (-1, 3), (-2, 4) }

We have obtained a new function.

However, this does not always work. Let f be the following set:

f = { (1, 2), (2, 4), (3, -1), (4, 2) }

Therefore g is:

g = { (2, 1), (4, 2), (-1, 3), (2, 4) }

which is not a function as g(2) is not determined by just one value; in other words g does not satisfy the condition of the function. There are two pairs of numbers, (2, 1) and (2, 4), whose first coordinate is the same and second coordinate is different.

What is the difference between these two examples? The reason is that in the second example f(1)=f(4)=2 and when the pairs are reversed g(2) is not determined by just one value; therefore g is not a function. In the first example the different values of "x" give different values of "y". Functions which behave in the same way as this first example are called injective or one-to-one.

 DEFINITION:  A function f is injective or one-to-one if f(a) is distinct to f(b) when a is distinct to b.

When another function is obtained by reversing the pairs of a function we can say that this function has an inverse. In the light of what we have stated above, the only functions which have inverse functions are injective functions.

 DEFINITION:  If f is an injective function, the inverse of f ,denoted by f-1 is the set:   f-1 = { (a, b) / (b, a) Î f } In other words, f-1 = { (x, y) / x=f(y), if y is the domain of f } = { (f(y), y) / if y is the domain of f }

From the definition we can see immediately that the domain of the inverse function f-1 is the range of f and, reciprocally, the range of f-1 is the domain of f. It is also easy to see that f-1(a)=b is the equivalent of f(b)=a. By using "x" and "y" which we often use when referring to functions f-1(x)=y is equivalent to f(y)=x. Another way of saying this is that: f(f-1(x))=x (where x belongs to the range of f), or, f-1(f(x))=x (where x belongs to the domain of f). By using the composition of functions, referring to the function defined by I(x)=x, I (Identity function) we can say that:

fof-1 = I        and        f-1of = I

except when the domain of the second member of these two equations is bigger than that of the first member if the domain of either f or f-1 is not all of R.

Incidentally, if a function has an inverse, what will (f-1)-1 be equal to? i.e. the inverse of the inverse function?

We have often referred to the idea of inverse functions in earlier units at this level. However, we have not labelled them as such. Think back to how we defined the square root, cube root etc.

The following window represents the graphs of the two function f and g from activity 2. The functions are indicated in blue and the reverse pairs of numbers in pink. The control point moves along the functions indicating a pair of the function and the corresponding pair of the inverse function. The other straight line that you can see (in green) bisects the first and third quadrant (the straight line of the equation y=x).

1.- Note that in order to determine whether a function has an inverse or not we must look at its pairs and see if it is an injective function. This is very straightforward when the function is given as a list of pairs. It is more complicated to do so if the function is defined by a property and we may not have enough mathematical knowledge to be able to decide (as was the case when we wanted to determine whether a certain set was a function or not).

If we have the graphical representation we can see that an injective function is defined graphically by the fact that no two points of the function are located on the same horizontal straight line. In other words, the graph of f is used to construct the graphical representation of the set of reverse pairs and this is used to see whether the set is a function or not.

2.- Analyse the following functions in the window:

1. The function f defined by y=2x-3, i.e., f = { (x, y) / y=2x-3 } = { (x, 2x-3) } has an inverse which is f-1 = { (y, x) / y=2x-3 } = { (x, y) / x=2y-3 } = { (2x-3, x) }
2. The function g defined by y=x2-2x-2, i.e., g = { (x, y) / y=x2-2x-2 } = { (x, x2-2x-2) } does not have an inverse. For example, the pairs (0, -2) and (2, -2) belong to g and therefore, g is not injective.

3.- Does the function h = { (x, y) / y=x2-2x-2, if x > 1 } = { (x, x2-2x-2) / x > 1 } have an inverse? Use the window above to answer the question correctly. If the answer is yes, show the graphical representation of the inverse in the window by using the control button in the window. Does the function k = { (x, y) / y=x2-2x-2, if x < 1 } have an inverse?

4.- Note that the graphs of a function and the set of reverse pairs are symmetrical about the straight line y=x (in green).

 4.  EXERCISES The following window shows three functions. 1.- Find out which functions have an inverse and which do not. 2.- Use the controls in the window to construct the set of reverse pairs for each function. 3.- What happens with the third function? What can you say about the graph with respect to the straight line y=x? Write a summary of your observations and prove them if possible.

 Salvador Calvo-Fernández Pérez Spanish Ministry of Education. Year 2001 