Definition:Real Number/Cauchy Sequences
Definition
Consider the set of rational numbers $\Q$.
For any two Cauchy sequences of rational numbers $X = \sequence {x_n}, Y = \sequence {y_n}$, define an equivalence relation between the two as:
- $X \equiv Y \iff \forall \epsilon \in \Q_{>0}: \exists n \in \N: \forall i, j > n: \size {x_i - y_j} < \epsilon$
A real number is an equivalence class $\eqclass {\sequence {x_n} } {}$ of Cauchy sequences of rational numbers.
Notation
While the symbol $\R$ is the current standard symbol used to denote the set of real numbers, variants are commonly seen.
For example: $\mathbf R$, $\RR$ and $\mathfrak R$, or even just $R$.
Also known as
When the term number is used in general discourse, it is often tacitly understood as meaning real number.
They are sometimes referred to in the pedagogical context as ordinary numbers, so as to distinguish them from complex numbers
However, depending on the context, the word number may also be taken to mean integer or natural number.
Hence it is wise to be specific.
Also see
- Equivalence Relation on Cauchy Sequences, which justifies the construction
- Results about real numbers can be found here.