linguistic:paraphrasing
Real numbers and topological properties

The rationals are a dense subset of the real numbers; every real number has rational numbers arbitrarily close to it. A related property is that rational numbers are the only numbers with finite expansions as regular continued fractions.

In the usual topology of the real numbers, the rationals are neither an open set nor a closed set.

By virtue of their order, the rationals carry an order topology. The rational numbers, as a subspace of the real numbers, also carry a subspace topology. The rational numbers form a metric space by using the absolute difference metric d (x, y) = | x − y |, {\displaystyle d(x,y)=|x-y|,} and this yields a third topology on Q. {\displaystyle \mathbb {Q}.} All three topologies coincide and turn the rationals into a topological field. The rational numbers are an important example of a space which is not locally compact. The rationals are characterized topologically as the unique countable metrizable space without isolated points. The space is also totally disconnected. The rational numbers do not form a complete metric space, and the real numbers are the completion of Q {\displaystyle \mathbb {Q} } under the metric d (x, y) = | x − y | {\displaystyle d(x,y)=|x-y|} above.