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MAT091 :: Lecture Note :: Week 03
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### Introduction to Functions

A function is a "relationship" that maps each input into one and only one output.

"Relationship" is quoted because functions can be defined in a variety of ways. The following was copied from the Wikipedia.

```   "The mathematical concept of a function expresses the
intuitive idea of deterministic dependence between two
quantities, one of which is viewed as primary (the independent
variable, argument of the function, or its 'input') and the
other as secondary (the value of the function, or 'output').
```

In the Wikipedia's function definition, the word deterministic implies that every input to a function always results in the same output.

##### Terminology

The valid inputs to a function represents the domain of the function.

The range of a function are all the possible outputs.

Inputs are independent of the outputs, but the outputs are dependent on the inputs.

Functions are typically given one letter names. In many instances, inputs are labeled (named) 'x' and outputs are labeled (named) 'y'.

```              +----------+
input ---> | function | ---> output
x        +----------+         y

y = f(x)   or    f(x) = y

y is the output, x is the input, f() is the function name
```

The following is a function with an implementation.

```   f(x) = 2x                   [each input is multiplied by 2]

f(-2) outputs -4            [input is -2]
f(-1) outputs -2            [input is -1]
f(0) outputs 0              [input is 0]
f(1) outputs 2              [input is 1]
f(2) outputs 4              [input is 2]

f() is a function because each input produces only one output.
Notice how each output value depends on the input value.

The domain of f() is all real numbers.
The range of f() is all real numbers.

Function f() could be named doubler() because the output
is always double (or 2 times) the input.

doubler(5) outputs 10
doubler(-10) outputs -20
doubler(5 * 3) outputs 30
doubler(1 - 2 - 3) outputs -8
```

Public domain dot-png from Wikipedia.org...

##### Are They Functions?
```   Is f() a potential function?  If yes, what's its domain and range?

+-------+
1 ----> |  f()  | ---->  -1          f(1) = -1            (1, -1)
+-------+
+-------+
2 ----> |  f()  | ---->  -2          f(2) = -2            (2, -2)
+-------+
+-------+
3 ----> |  f()  | ---->  -3          f(3) = -3            (3, -3)
+-------+

Is g() a potential function?  If yes, what's its domain and range?

+-------+
1 ----> |  g()  | ---->  -1          g(1) = -1            (1, -1)
+-------+
+-------+
2 ----> |  g()  | ---->  -2          g(2) = -2            (2, -2)
+-------+
+-------+
3 ----> |  g()  | ---->  -2          g(3) = -2            (3, -2)
+-------+

Is h() a potential function?  If yes, what's its domain and range?

+-------+
1 ----> |  h()  | ---->  -1          h(1) = -1            (1, -1)
+-------+
+-------+
2 ----> |  h()  | ---->  -2          h(2) = -2            (2, -2)
+-------+
+-------+
2 ----> |  h()  | ---->  -3          h(2) = -3            (2, -3)
+-------+
+-------+
3 ----> |  h()  | ---->  -4          h(3) = -4            (3, -4)
+-------+

Is Q() a potential function?  If yes, what's its domain and range?

+-------+
1 ----> |  Q()  | ---->  -1          Q(1) = -1            (1, -1)
+-------+
+-------+
2 ----> |  Q()  | ---->  -2          Q(2) = -2            (2, -2)
+-------+
+-------+
2 ----> |  Q()  | ---->  -2          Q(2) = -2            (2, -2)
+-------+
+-------+
3 ----> |  Q()  | ---->  -3          Q(3) = -3            (3, -3)
+-------+
```

If a function receives an input that is not in its domain, then the output of the function is "undefined." (i.e. A function does not work on inputs that are not in its domain.) If the domain of a function is not stated, then the its domain is all real numbers.

##### Related BABs and External Hyperlink(s)

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### Quote From ASU Math Professors

Marilyn Carlson, Michael Oehrtman and Patrick W. Thompson of Arizona State University have authored a 21 page dot-pdf titled: "Foundational Reasoning Abilities that Promote Coherence in Students' Understanding of Function." Their paper starts as follows.

```   "The concept of function is central to undergraduate mathematics,
foundational to modern mathematics, and essential in related
areas of the sciences."

"Since 1888, there have been repeated calls for school curricula
to place greater emphasis on functions."
```

Let's look at yet another definition for a mathematical function.

```   "A relation for which each element of the domain corresponds
to exactly one element of the range."
```

Some function examples.

```   current world population is a function of time
category of a hurricane is a function of wind speed
cost of a loan is a function of its interest rate
calories consumed is a function of serving size
diameter of a circle is a function of its radius
```

Beginning algebra courses focus on function having one independent variable (i.e. one input), but there are many functions that depend on multiple inputs.

```   wind chill is a function of temperature & wind speed
cost to gas a car is a function of #gallons pumped & cost per gallon
perimeter of a rectangle is a function of its length & width
duration of a road trip is a function of distance & avg. speed
total change collected is a function of #half-dollars, #quarters,
#dimes, #nickels & #pennies
```

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### Terminology Related To Functions

A function takes one input value and produces one output value. The same input value always produces the same output value.

```   input     domain    indepedent     horizontal-axis     x-axis
output    range     dependent      vertical-axis       y-axis
```

If a specific domain is not given for a function, then its domain is all real numbers.

If a function receives an input that is not in its domain, then the output of the function is undefined (i.e. the function doesn't work).

The output depends on the input. The input is independent of the output.

For a given domain, a function always produces the same range.

```   f(x) is read "f of x"     [not f times x]
g(n) is read "g of n"     [not g times n]
h(t) is read "h of t"     [not h times t]

f(x), g(n), h(t) all represent an output value

f(x) is a function of x   [f(x) depends on x]
g(n) is a function of n   [g(n) depends on n]
h(t) is a function of t   [h(t) depends on t]
```

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### Constant Function

A constant function is a function that produces the same output regardless of the input.

```   f(x) = k, where k is a constant
```

The graph of a constant function is a horizontal line.

If zero is in the function's domain, then the constant function intersects the vertical-axis at its constant output value.

```   f(x) = 3

f(-2) = 3
f(0) = 3
f(2) = 3

The graph for f() is a horizontal line that intersects
the vertical-axis at the point (0,3).

| 5
|
A------B------C        A: (-2,3)   B: (0,3)   C: (2,3)
|
|
-------+-------
-2     |      2
|
|
|
| -5
```

Unless stated otherwise, the domain of a constant function is all real numbers. The range of the constant function is the constant value that it always outputs.

##### Example of a Constant Functon

The following is the Speed At Stop Sign Function (SASSF).

```   f(s) = 0 MPH

's'  is the speed at which a vehicle approaches a stop sign
f(s) is the speed a vehicle should be moving at a stop sign

's' and f(s) are both in units of MPH (Miles Per Hour)

domain:  0 < s < infinity
range:  0

note:  For this specific constant function, the output does
not depend on the input.

note:  As of 8 June 2007, the land speed record is 763 MPH;
therefore, the practical domain for this function
could be 0 < s < 763.
```
##### Constant Functions are Linear Functions

A constant function is a linear function having zero slope.

```   f(x) = mx + b for a constant function is f(x) = 0x + b
```

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### Identity Function

A identity function is a function that returns its input for all valid inputs.

```   f(x) = x
```

The graph of an identity function is straight line having a slope of one.

If zero is in the function's domain, then the indentity function intersects the vertical-axis at point (0,0).

```   f(x) = x              [implementation]

f(-2) = -2
f(-1) = -1
f(0) = 0
f(1) = 1
f(2) = 2

The graph for f() is a straight line having a slope of
one that intersects the vertical-axis at the point (0,0).

| 5
|
|
|     E             E: (2, 2)
-2     |  D                D: (1, 1)
------C------             C: (0, 0)
B  |     2             B: (-1, -1)
A     |                   A: (-2, -2)
|
|
| -5
```

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### Absolute Value Function

The absolute value function outputs the absolute value of its input.

```   f(x) = |x|
```

The graph of an absolute value function, where the domain is all real numbers, is two staight lines that intersect at point (0,0). There is a straight line in the 2nd quadrant that has a slope of -1 and there is a straight line in the 1st quadrant having a slope of 1.

```   f(x) = |x|

f(-4) = 4
f(-2) = 2
f(0) = 0
f(2) = 2
f(4) = 4

| 5
A            |            E          A: (-4,4)    E: (4,4)
|
B      |      D                B: (-2,2)    D: (2,2)
|
-------------C-------------          C: (0,0)
-4            |            4
|
|
|
| -5
```

If the domain of an absolute value function is all real numbers, then its range is all real numbers greater than or equal to zero.

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### Ordered Pairs

The inputs and outputs of a function are often represented as ordered pairs (or points).

```   (input, output) ...or... (x, y) ...or... (x, f(x))
(t, n) ...or... (t, f(t))
(n, o) ...or... (n, f(n))
(a, b) ...or... (a, f(a))
```

The input is always the first value recorded followed by its respective output.

Data contained in tables can sometimes be represented as ordered-pairs.

```   input:  month ......... output:  #days

month |  1 |  2 |  3 |  4 |  5 |  6 |  7 |  8 |  9 | 10 | 11 | 12
------------------------------------------------------------------
#days | 31 | 28 | 31 | 30 | 31 | 30 | 31 | 31 | 30 | 31 | 30 | 31

(1, 31)   (2, 28)  (3, 31)  (4, 30)   (5, 31)  (6, 30)
(7, 31)   (8, 31)  (9, 30)  (10, 31)  (11,30)  (12, 31)

#days depends on the month (i.e. the output depends on the input)

The function  f(m)  takes a month as input and outputs
the number of days in that month.

f(m) = n

f(1) = 31
f(2) = 28
f(7) = 31
f(12) = 31
```

Ordered-pairs are points that can be graphed. The input is along the horizontal-axis and output is along the vertical-axis.

The following ordered-pairs could represent a function because all of the inputs are unique (i.e. different).

```   (-2, 3)  (0, 4)  (5, -2)  (7, 4)  (11, 7)
```

The following ordered-pairs could represent a function because all of the inputs are unique although some of the inputs repeat.

```   (-2, 3)  (0, 4)  (5, -2)  (7, 4)  (11, 7)  (0, 4)
```

The following ordered-pairs cannot be a function because input 3 produces different outputs (2 and 5).

```   (-2, 3)  (0, 4)  (-2, 3)  (3, 2)  (0, 4)  (3, 5)
```

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The graph formed by the intersection of a horizontal line and a vertical line can be divided into four quadrants.

```      vertical-axis (y-axis; outputs)
|
2 (II)   |     1 (I)
|
-----------o----------- horizontal-axis (x-axis; inputs)
|                  o is the origin (0, 0)
3 (III)  |     4 (IV)
|
```

The top-right quadrant is always the first quadrant with the remining quadrants numbered in a counter-clockwise direction.

Quadrants are sometimes numbered (labeled) using Roman Numerals.

Ordered-pairs (i.e. points or coordinates) are plotted (graphed) as follows.

```   (+x, +y) ... quadrant 1 (I)
(-x, +y) ... quadrant 2 (II)
(-x, -y) ... quadrant 3 (III)
(+x, -y) ... quadrant 4 (IV)
( 0,  0) ... origin
( 0, +y) ... vertical axis; above the origin
( 0, -y) ... vertical axis; below the origin
(+x,  0) ... horizontal axis; right of origin
(-x,  0) ... horizontal axis; left of origin
```

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