# Set of Numbers

## Set of Numbers

A set in which all of the elements are numbers.

### Notations

The different sets of numbers are identified by letters :

### Historical Note

We may well wonder where the choice of the letters to describe the different sets of numbers comes from. Here are a few explanations :

• In his work on the axiomatization of the set of non-zero integers, the Italian mathematician Giuseppe Peano used the capital letter N, for the Italian word naturale, which later became $$\mathbb{N}$$, to indicate the set of non-zero whole numbers. He also used the letter $$\mathbb{Q}$$, for the first letter of the Italian word quotient which means “quotient”, to refer to the set of rational numbers.
• In the works of the Bourbaki group (around 1970), they used the letter $$\mathbb{D}$$ to indicate the set of decimal numbers and $$\mathbb{Z}$$ to refer to the set of integers (for the German word zahlen which means “to count”. This may also be an idea from the German mathematician Dedekind.
• The choice of the letter $$\mathbb{R}$$ to refer to the set of real numbers comes from Julius Wilhelm Dedekind (1831-1916), a German mathematician, who introduced it in his texts.
• Dedekind also discussed complex numbers and algebraic numbers in his texts, but the choices of the letters $$\mathbb{C}$$ for complex numbers and $$\mathbb{\overline{Q}}$$ for algebraic numbers are more recent although we find them in many texts.