# Topological field

A topological ring $K$ that is a field, where in addition it is required that the inverse mapping $a\mapsto a^{-1}$ is continuous on $K\setminus \{0\}$. Any subfield $P$ of a topological field $K$, and the closure $\overline{P}$ of $P$ in $K$, is also a topological field.
The only connected locally compact topological fields are and (cf. also Locally compact skew-field). Every normed field is a topological field with respect to the topology induced by the norm (cf. Norm; Norm on a field). If there exist two real-valued norms and on a field , each of which makes a complete topological field, and if the topologies and induced by and are distinct, then the field is algebraically closed. The field is the unique real-normed extension of the field .
On every field of infinite cardinality there exist precisely distinct topologies which make it a topological field. The topology of a topological field is either anti-discrete or completely regular. Topological fields with non-normal topologies, as well as topological fields with a normal but not hereditarily-normal topology, have been constructed. Topological fields are either connected or totally disconnected. There exists a connected topological field of arbitrary finite characteristic. It is unknown (1992) whether every topological field can be imbedded as a subfield in a connected topological field. In contrast to topological rings and linear topological spaces, not every completely-regular topological space can be imbedded as a subspace of a topological field. For example, a pseudo-compact (in particular, compact) subspace of a topological field is always metrizable. However, every completely-regular space admitting a continuous bijection onto a metric space can be imbedded as a subspace in some topological field. If the topological field contains at least one countable non-closed subset, then there exists a weakest metrizable topology on turning it into a topological field.
For a topological field one defines the completion — a complete topological ring in which is imbedded as an everywhere-dense subfield. The ring can have zero divisors. However, the completion of every real-normed topological field is a real-normed topological field.