# Number Systems: Real, Rational, Integer

When we're working in mathematics, we often want to make reference to different types of numbers.

We'll often see and use set notation as a shorthand, with key sets written as lackboard old letters^{[1]}.

Useful types of numbers to know and recognise:

- - the Real Numbers
- - the Rational Numbers
- - the Natural Numbers
- - the Integers
- - the Positive Integers
- - the Non-Negative Integers
- - the Complex Numbers

## Real Numbers

This refers to all numbers that can be found on a normal number line. So, it includes ..., i.e. all of the numbers.

The Real Numbers likely includes all numbers that you have studied, unless you have reached imaginary and complex numbers.

To talk about the real numbers, we'll write , and read this as " is a member of the Real Numbers" or " belongs to the Real Numbers".

## Rational Numbers

The letter refers to the word 'quotient', the result of a division - in other words, a fraction. Rational numbers are numbers that can be written as a fraction.

But be careful about this! Only whole numbers are allowed in the fraction.

Therefore, irrational numbers like won't count. You can't write as a whole number over a whole number.

Note that integers *do* count as rational. , so any whole number can be written as a fraction.

Note that zero does also count as rational! . We are always allowed on the top of a fraction.

All numbers in can be written as where and .

We'll write , and say " is a rational number".

## Natural Numbers

** Warning:** Natural Numbers and have more than one possible definition, which can be confusing.

The "Natural" numbers are also called counting numbers:

We'll write , often using in preference , to help remind us that we are talking about a natural number or integer.

Many mathematicians (including teachers) may prefer to start the Natural numbers with 0 instead of starting with 1, or this may depend on context. So, this can sometimes be unclear.

If you see in an official A-Level question, they are starting with 1, the same as .

I recommend avoiding this language and notation, where possible, and using a variation of instead.

## Integers

This refers to all whole numbers, including positive integers, negative integers and zero.

So, we have all the numbers:

We'll write and say " is an integer" or " belongs to the integers". It's nice to use to remind us that we're talking only about an integer, and not any other real number.

## Positive Integers

This refers clearly to only *positive* integers - the numbers

Zero is in its own category: neither positive nor negative, so it doesn't belong here in .

If it's especially important to remember that zero doesn't belong, saying "*strictly* positive integers" can act as a reminder to exclude zero.

## Non-Negative Integers

** Warning:** The notation can be unclear, but *non-negative* does have a clear meaning.

means the positive integers together with zero. So, the numbers:

We can write and say " is a *non-negative* integer".

I find it clearer to write this as two statements with a comma: .

## Complex Numbers

Complex Numbers are an extension of the number line into a two dimensional set of axes.

Their existence follows from the definition of the imaginary number, , which is first seen in Further Maths A-Level.

We'll write , usually using in place of , to help remind us that we are taking about complex numbers.

We can also say that a complex number , where

If we need a second complex number, we'll often use and write , or: , where

## Footnotes

- β Blackboard Bold is designed to be easy to write - you draw the first vertical line in the capital letter twice