topology (point-set topology, point-free topology)
see also differential topology, algebraic topology, functional analysis and topological homotopy theory
Basic concepts
fiber space, space attachment
Extra stuff, structure, properties
Kolmogorov space, Hausdorff space, regular space, normal space
sequentially compact, countably compact, locally compact, sigma-compact, paracompact, countably paracompact, strongly compact
Examples
Basic statements
closed subspaces of compact Hausdorff spaces are equivalently compact subspaces
open subspaces of compact Hausdorff spaces are locally compact
compact spaces equivalently have converging subnet of every net
continuous metric space valued function on compact metric space is uniformly continuous
paracompact Hausdorff spaces equivalently admit subordinate partitions of unity
injective proper maps to locally compact spaces are equivalently the closed embeddings
locally compact and second-countable spaces are sigma-compact
Theorems
Analysis Theorems
symmetric monoidal (∞,1)-category of spectra
A topological ring is a ring internal to Top, a ring object in Top:
a topological space $R$ equipped with the structure of a ring on its underlying set, such that addition $+$ and multiplication $\cdot$ are continuous functions.
Remarks:
The structure of a topological ring $(R,+,\cdot)$ makes $R$ a uniform space.
It is automatic that negation in a topological unital ring is continuous, since it is the operation of multiplication with $-1$, and multiplication is continuous in each argument. Hence for $(R,+,\cdot)$ a topological ring, then $(R,+)$ is a topological group.
A topological field is a topological ring $K$ whose underlying ring is in fact a field and such that reciprocation $(-)^{-1}: K \setminus \{0\} \to K \setminus \{0\}$ is continuous. This latter condition is the same as demanding that the subspace topology on $K \setminus \{0\}$ induced by the embedding $K \setminus \{0\} \hookrightarrow K$ coincide with the subspace topology induced by the embedding $K \setminus \{0\} \to K \times K: x \mapsto (x, x^{-1})$. More at topological field.
In a topological ring, the closure of $\{0\}$ is an ideal. It follows that for a topological field $F$, either $0$ is a closed point (so that $F$ is $T_1$ and therefore completely regular Hausdorff, by standard arguments in the theory of uniform spaces), or is a codiscrete space.
A topological algebra over a topological ring $R$ is a topological ring $S$ together with a topological ring map $R \to S$ that makes $S$ an $R$-algebra at the underlying set level (a topological associative algebra).
The real numbers form a topological field.
Any pseudocompact ring such as the completed group ring of a profinite group is a topological ring.
For any prime $p$, the ring of p-adic integers is a topological ring.
A Banach algebra is in particular a topological algebra, hence a topological ring. Hence so is a C-star-algebra.
Lecture notes include
See also
Wikipedia, Topological ring
Last revised on May 30, 2017 at 09:42:30. See the history of this page for a list of all contributions to it.