What is a Function?

A function relates an input to an output.

function cogs

It is like a machine that has an input and an output.

And the output is related somehow to the input.


  f(x)  

"f(x) = ... " is the classic way of writing a function.
And there are other ways, as you will see!

Input, Relationship, Output

We will see many ways to think about functions, but there are always three main parts:

Example: "Multiply by 2" is a very simple function.

Here are the three parts:

Input Relationship Output
0 × 2 0
1 × 2 2
7 × 2 14
10 × 2 20
... ... ...

For an input of 50, what is the output?

Some Examples of Functions

But we are not going to look at specific functions ...
... instead we will look at the general idea of a function.

Names

First, it is useful to give a function a name.

The most common name is "f", but we can have other names like "g" ... or even "marmalade" if we want.

But let's use "f":

f(x) = x^2

We say "f of x equals x squared"

what goes into the function is put inside parentheses () after the name of the function:

So f(x) shows us the function is called "f", and "x" goes in

And we usually see what a function does with the input:

f(x) = x2 shows us that function "f" takes "x" and squares it.

 

Example: with f(x) = x2:

In fact we can write f(4) = 16.

 

The "x" is Just a Place-Holder!

Don't get too concerned about "x", it is just there to show us where the input goes and what happens to it.

It could be anything!

So this function:

f(x) = 1 - x + x2

Is the same function as:

The variable (x, q, A, etc) is just there so we know where to put the values:

f(2) = 1 - 2 + 22 = 3

 

Sometimes There is No Function Name

Sometimes a function has no name, and we see something like:

y = x2

But there is still:

Relating

At the top we said that a function was like a machine. But a function doesn't really have belts or cogs or any moving parts - and it doesn't actually destroy what we put into it!

A function relates an input to an output.

Saying "f(4) = 16" is like saying 4 is somehow related to 16. Or 4 → 16

tree

Example: this tree grows 20 cm every year, so the height of the tree is related to its age using the function h:

h(age) = age × 20

So, if the age is 10 years, the height is:

h(10) = 10 × 20 = 200 cm

Here are some example values:

age h(age) = age × 20
0 0
1 20
3.2 64
15 300
... ...

 

What Types of Things Do Functions Process?

"Numbers" seems an obvious answer, but ...

calculator

... which numbers?

For example, the tree-height function h(age) = age×20 makes no sense for an age less than zero.

codes ... it could also be letters ("A"→"B"), or ID codes ("A6309"→"Pass") or stranger things.

So we need something more powerful, and that is where sets come in:

various real numbers

A set is a collection of things.

Here are some examples:

  • Set of even numbers: {..., -4, -2, 0, 2, 4, ...}
  • Set of clothes: {"hat","shirt",...}
  • Set of prime numbers: {2, 3, 5, 7, 11, 13, 17, ...}
  • Positive multiples of 3 that are less than 10: {3, 6, 9}

Each individual thing in the set (such as "4" or "hat") is called a member, or element.

So, a function takes elements of a set, and gives back elements of a set.

A Function is Special

But a function has special rules:

This can be said in one definition:

function sets X to Y

Formal Definition of a Function

A function relates each element of a set
with exactly one element of another set
(possibly the same set).

 

The Two Important Things!

1.

"...each element..." means that every element in X is related to some element in Y.

We say that the function covers X (relates every element of it).

(But some elements of Y might not be related to at all, which is fine.)

2.

"...exactly one..." means that a function is single valued. It will not give back 2 or more results for the same input.

So "f(2) = 7 or 9" is not right!

"One-to-many" is not allowed, but "many-to-one" is allowed:

function   function
(one-to-many)   (many-to-one)
This is NOT OK in a function   But this is OK in a function

When a relationship does not follow those two rules then it is not a function ... it is still a relationship, just not a function.

Example: The relationship x → x2

function

Could also be written as a table:

X: x Y: x2
3 9
1 1
0 0
4 16
-4 16
... ...

It is a function, because:

So it follows the rules.

(Notice how both 4 and -4 relate to 16, which is allowed.)

Example: This relationship is not a function:

function

It is a relationship, but it is not a function, for these reasons:

(But the fact that "6" in Y has no relationship does not matter)

 

function not single valued

Vertical Line Test

On a graph, the idea of single valued means that no vertical line ever crosses more than one value.

If it crosses more than once it is still a valid curve, but is not a function.

Some types of functions have stricter rules, to find out more you can read Injective, Surjective and Bijective

Infinitely Many

My examples have just a few values, but functions usually work on sets with infinitely many elements.

Example: y = x3

We can't show ALL the values, so here are just a few examples:

X: x Y: x3
-2 -8
-0.1 -0.001
0 0
1.1 1.331
3 27
and so on... and so on...

 

Domain, Codomain and Range

In our examples above

We have a special page on Domain, Range and Codomain if you want to know more.

So Many Names!

Functions have been used in mathematics for a very long time, and lots of different names and ways of writing functions have come about.

Here are some common terms you should get familiar with:

Function Parts

Example: z = 2u3:

Example: f(4) = 16:

Example: h(year) = 20 × year:

eq

We often call a function "f(x)" when in fact the function is really "f"

Ordered Pairs

And here is another way to think about functions:

Write the input and output of a function as an "ordered pair", such as (4,16).

They are called ordered pairs because the input always comes first, and the output second:

(input, output)

So it looks like this:

( x, f(x) )

Example:

(4,16) means that the function takes in "4" and gives out "16"

Set of Ordered Pairs

A function can then be defined as a set of ordered pairs:

Example: {(2,4), (3,5), (7,3)} is a function that says

"2 is related to 4", "3 is related to 5" and "7 is related 3".

Also, notice that:

But the function has to be single valued, so we also say

"if it contains (a, b) and (a, c), then b must equal c"

Which is just a way of saying that an input of "a" cannot produce two different results.

Example: {(2,4), (2,5), (7,3)} is not a function because {2,4} and {2,5} means that 2 could be related to 4 or 5.

In other words it is not a function because it is not single valued

 

interactive-cartesian-coordinates

A Benefit of Ordered Pairs

We can graph them...

... because they are also coordinates!

So a set of coordinates is also a function (if they follow the rules above, that is)

 

A Function Can be in Pieces

We can create functions that behave differently depending on the input value

Example: A function with two pieces:

Piecewise Function   Here are some example values:
x y
-3 5
-1 5
0 0
2 4
4 16
... ...

Read more at Piecewise Functions.

Explicit vs Implicit

One last topic: the terms "explicit" and "implicit".

Explicit is when the function shows us how to go directly from x to y, such as:

y = x3 − 3

When we know x, we can find y

That is the classic y = f(x) style that we often work with.

Implicit is when it is not given directly such as:

x2 − 3xy + y3 = 0

When we know x, how do we find y?

It may be hard (or impossible!) to go directly from x to y.

"Implicit" comes from "implied", in other words shown indirectly.

Graphing

Conclusion