Real line

The real line

In mathematics, the real line is the line whose points correspond to the real numbers. That is, the real line is the set R of all real numbers, viewed as a geometric space. It is the Euclidean space of dimension one, and can be thought of as a vector space (or affine space), a metric space, a topological space, or simply as a linear continuum.

Just like the set of real numbers, the real line is usually denoted by the symbol R (or alternatively, , the letter “R” in blackboard bold). However, it is sometimes denoted R¹ in order to emphasize its role as the first Euclidean space.

The real line carries a standard topology which can be introduced in two different, equivalent ways. First, since the real numbers are totally ordered, they carry an order topology. With respect to this topology, the real line is a linear continuum. Second, the real numbers can be turned into a metric space by using the metric given by the absolute value

This metric induces a topology on R equivalent to the order topology.

The real line is trivially a topological manifold of dimension 1. It is paracompact and second-countable as well as contractible and locally compact. It also has a standard differentiable structure on it, making it a differentiable manifold. (Up to diffeomorphism, there is only one differentiable structure that the topological space supports.) Indeed, R was historically the first example to be studied of each of these mathematical structures, so that it serves as the inspiration for these branches of modern mathematics. (Many of the terms above can't even be defined until R is already in place.)

As a vector space, the real line is a vector space over the field R of real numbers (that is, over itself) of dimension 1. It has a standard inner product, making it a Euclidean space. (The inner product is simply ordinary multiplication of real numbers.) As a vector space, it is not very interesting, and thus it was in fact 2-dimensional Euclidean space that was first studied as a vector space. However, we can still say that R inspired the field of linear algebra, since vector spaces were first studied over R.

R is also a premier example of a ring, even a field. It is in fact a real complete field, and was the first such field to be studied, so that it inspired that branch of abstract algebra as well. However, in such purely algebraic contexts, R is rarely called a "line".

For more information on R in all of its guises, see real number.

See also

• Imaginary line (mathematics)
• Number line