# 2. Number Properties

This section gives an overview of the properties of various numbers that you'll meet during your math journey.

## Real and Imaginary Numbers

Most of the time we’ll come across **real numbers** only. Real numbers include natural numbers (or "whole" numbers),
negative numbers, integers, fractions, decimals, square roots and special numbers like π.

However, our number system consists of **real numbers** and **imaginary numbers**.

### Numbers

**Real Numbers**

**Imaginary Numbers**

But what are these "un-real numbers"? **Imaginary numbers** involve the square root of a negative number. We meet
imaginary numbers later in the Complex Numbers chapter, and see how they are used in
electronics.

## Rational and Irrational Numbers

A real number is either **rational** or **irrational.**

### Numbers

**Real Numbers**

**Rational Numbers**

**Irrational Numbers**

**Imaginary Numbers**

A number is **rational** if it can be expressed in the form `p/q`, where *p* and *q* are both integers.

For example,

`3/5` is

rational, since `3` and `5` are integers.

Other examples of rational numbers are:

`1 2/3` (it is equal to `5/3`),

`7,`

`0,`

`8.25`

Rational numbers either have a

**terminating decimal**(that is, the decimal stops, e.g. `7.625`) or**repeating decimal**(e.g. `2/99 = 0.02020202...` is a rational number).

### Irrational numbers

π = 3.14159... is
**irrational**, since it cannot be expressed in the form `p/q`.

(BTW, π does NOT equal `22/7`. This is just a handy approximation.)

An irrational number has a never ending (and never repeating) decimal.

Other irrational numbers are:

e= 2. 718 281 828 ... (For an explanation ofe, see Natural Logarithms)√3 = 1.732 05...

Φ = 1.618 033... ("phi") (see Math of Beauty for some interesting examples)

## Summary of Number Types

### Numbers

**Real Numbers**

**Rational Numbers**

Can be expressed as

*p*/

*q*

(

*p*,

*q*integers)

**Includes:**

Fractions, decimals, natural numbers

**Integers**

Negative (...,-3,-2,-1)

0

Positive (1,2,3,...)

Prime numbers, composite numbers

**Irrational Numbers**

Cannot be expressed as

*p*/

*q*

(

*p*,

*q*integers)

**Examples:**

π = 3.141592..., *
e* = 2. 718 281...,

√3 = 1.7321...

**Imaginary Numbers**

Complex numbers have a

**real**part and an

**imaginary**part.

Read more in the Complex Numbers chapter.

**Example:** 7 + 2*j*

## Prime Number Properties

A **prime number** is a positive integer which has exactly two factors, 1 and itself. The first primes are:

2, 3, 5, 7, 11, 13, ...

The other positive integers are **composite** and they have 3 or more factors. For example,

9 has factors 1, 3 and 9 (3 factors), so is composite.

12 has factors 1, 2, 3, 4, 6, 12 (6 factors), so is composite.

The first composite numbers are:

4, 6, 8, 9, 10, 12, ...

So the positive integers consists of the following:

### Positive Integers

**Not prime,**

not composite

not composite

1

**Prime Numbers**

2, 3, 5, 7, ...

**Composite Numbers**

4, 6, 8, 9, ...

The number 1 is neither prime nor composite, since it has exactly one factor.

## Who cares about Prime Numbers?

A lot of energy has been expended by mathematicians (ever since the time of Pythagoras) on studying prime numbers.

Recently, a very important branch of mathematics to emerge is **encryption**, where sensitive information is hidden from others
when it is transmitted electronically (e.g. when we send credit card numbers over the Internet or by mobile phone).

Encryption works by coding the message using very large prime numbers. The device receiving the message decodes the message using the same very large prime numbers. The larger the numbers used, the better the encryption.

The largest known prime currently is

2

^{43,112,609}− 1 (This is huge - it has almost 13 million digits).

## Reciprocal of a Number

The **reciprocal** of a number *x* is `1/x`. (In plain English, you turn the fraction upside-down).

### Examples of Reciprocals

The reciprocal of `1/5` is 5.

The reciprocal of `3/5` is `5/3`, which equals `1 2/3`.

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