Topological algebra

In mathematics, a topological algebra $A$ is an algebra and at the same time a topological space, where the algebraic and the topological structures are coherent in a specified sense.

Definition

A topological algebra $A$ over a topological field $K$ is a topological vector space together with a bilinear multiplication

$\cdot :A\times A\longrightarrow A$,

$(a,b)\longmapsto a\cdot b$

that turns $A$ into an algebra over $K$ and is continuous in some definite sense. Usually the continuity of the multiplication is expressed by one of the following (non-equivalent) requirements (ordered according to the successive weakening of requirements):

• joint continuity: for each neighbourhood of zero $U\subseteq A$ there are neighbourhoods of zero $V\subseteq A$ and $W\subseteq A$ such that $V\cdot W\subseteq U$ (in other words, this condition means that the multiplication is continuous as a map between topological spaces $A\times A\longrightarrow A$), or

• stereotype continuity: for each totally bounded set $S\subseteq A$ and for each neighbourhood of zero $U\subseteq A$ there is a neighbourhood of zero $V\subseteq A$ such that $S\cdot V\subseteq U\quad \&\quad V\cdot S\subseteq U$.

• separate continuity: for each element $a\in A$ and for each neighbourhood of zero $U\subseteq A$ there is a neighbourhood of zero $V\subseteq A$ such that $a\cdot V\subseteq U$ and $V\cdot a\subseteq U$.

In the first case $A$ is called a topological algebra with jointly continuous multiplication, and in the third - with separately continuous multiplication.

A unital associative topological algebra is (sometimes) called a topological ring.

History

The term was coined by David van Dantzig; it appears in the title of his doctoral dissertation (1931).

Examples

1. Fréchet algebras are examples of associative topological algebras with jointly continuous multiplication.

2. Banach algebras are special cases of Fréchet algebras.

3. Stereotype algebras are examples of associative topological algebras with stereotype continuous multiplication.

External links

• Topological algebra at the nLab

References

• Beckenstein, E.; Narici, L.; Suffel, C. (1977). Topological Algebras. Amsterdam: North Holland. ISBN 9780080871356. {{cite book}}: Invalid |ref=harv (help)

• Akbarov, S.S. (2003). "Pontryagin duality in the theory of topological vector spaces and in topological algebra". Journal of Mathematical Sciences. 113 (2): 179–349. doi:10.1023/A:1020929201133. {{cite journal}}: Invalid |ref=harv (help)

• Mallios, A. (1986). Topological Algebras. Amsterdam: North Holland. ISBN 9780080872353. {{cite book}}: Invalid |ref=harv (help)