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Dense submodule : ウィキペディア英語版
Dense submodule
In abstract algebra, specifically in module theory, a dense submodule of a module is a refinement of the notion of an essential submodule. If ''N'' is a dense submodule of ''M'', it may alternatively be said that "''N'' ⊆ ''M'' is a rational extension". Dense submodules are connected with rings of quotients in noncommutative ring theory. Most of the results appearing here were first established in , and .
It should be noticed that this terminology is different from the notion of a dense subset in general topology. No topology is needed to define a dense submodule, and a dense submodule may or may not be topologically dense in a module with topology.
== Definition ==
This article modifies exposition appearing in and . Let ''R'' be a ring, and ''M'' be a right ''R'' module with submodule ''N''. For an element ''y'' of ''M'', define
:y^N=\ \,
Note that the expression ''y''−1 is only formal since it is not meaningful to speak of the module-element ''y'' being invertible, but the notation helps to suggest that ''y''⋅(''y''−1''N'') ⊆ ''N''. The set ''y'' −1''N'' is always a right ideal of ''R''.
A submodule ''N'' of ''M'' is said to be a dense submodule if for all ''x'' and ''y'' in ''M'' with ''x'' ≠ 0, there exists an ''r'' in ''R'' such that ''xr'' ≠  and ''yr'' is in ''N''. In other words, using the introduced notation, the set
:x(y^N)\neq\ \,
In this case, the relationship is denoted by
:N\subseteq_d M\,
Another equivalent definition is homological in nature: ''N'' is dense in ''M'' if and only if
:\mathrm_R (M/N,E(M))=\\,
where ''E''(''M'') is the injective hull of ''M''.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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