Numbers can be classified into different sets. Here are the most useful sets: |

are natural numbers. |
aren't natural numbers. |

is the set of relative numbers, or integers. They are numbers without point.

are integers. |
aren't integers. |

is the set of decimal numbers. They are numbers with a finite number of figures after the point.

are decimal numbers. |
aren't decimal numbers. |

is the set of rational numbers. They are the numbers you can write in the form of fractions.

are rational numbers. |
aren't rational numbers. |

is the set of real numbers. Every number you know is a real number.

is the set of positive numbers.

is the set of negative numbers.

is the set of numbers but without zero.

The number -4 belongs to the set of relative numbers. You can also write : . The same way, , and

.

[4;5[ is the set of all numbers between 4 and 5, except 5.

]-2;1[ is the set of all numbers between -2 and 1, except -2 and 1.

You symbolize the union by a capital u : U.

[1;2]U[4;5]

[0;4]U[2;5] = [0;5]

]-4;-2]U]-3;-1] = ]-4 ; -1]

[0;4]U[2;5] = [0;5]

]-4;-2]U]-3;-1] = ]-4 ; -1]

The last result is the empty set, that is the set without any number in it.