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Direct sum of topological groups : ウィキペディア英語版
Direct sum of topological groups
In mathematics, a topological group ''G'' is called the topological direct sum〔E. Hewitt and K. A. Ross, Abstract harmonic analysis. Vol. I, second edition, Grundlehren der Mathematischen Wissenschaften, 115, Springer, Berlin, 1979. MR0551496 (81k:43001)〕 of two subgroups ''H''1 and ''H''2 if
the map
:\begin
H_1\times H_1 &\longrightarrow G \\
(h_1,h_2) &\longmapsto h_1 h_2
\end

is a topological isomorphism.
More generally, ''G'' is called the direct sum of a finite set of subgroups H_i, i=1,\ldots, n of the map
:\begin
\prod^n_ H_i&\longrightarrow G \\
(h_i)_ &\longmapsto h_1 h_2 \cdots h_n
\end

Note that if a topological group ''G'' is the topological direct sum of the family of subgroups H_i then in particular, as an abstract group (without topology) it is also the direct sum (in the usual way) of the family H_i .
==Topological direct summands==
Given a topological group ''G'', we say that a subgroup ''H'' is a topological direct summand of ''G'' (or that splits topologically form ''G'' ) if and only if there exist another subgroup ''K'' ≤ ''G'' such that ''G'' is the direct sum of the subgroups ''H'' and ''K''.
A the subgroup ''H'' is a topological direct summand if and only if the extension of topological groups
:0 \to H\stackrel G\stackrel G/H\to 0
splits, where i is the natural inclusion and \pi is the natural projection.

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