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Semi-invariant of a quiver
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Semi-invariant of a quiver : ウィキペディア英語版
Semi-invariant of a quiver
In mathematics, given a quiver Q with set of vertices Q0 and set of arrows Q1, a representation of Q assigns a vector space ''V''''i'' to each vertex and a linear map ''V''(''α''): ''V''(''s''(''α'')) → ''V''(''t''(''α'')) to each arrow ''α'', where ''s''(''α''), ''t''(''α'') are, respectively, the starting and the ending vertices of α. Given an element d ∈ ℕQ0, the set of representations of Q with dim ''V''''i'' = d(i) for each ''i'' has a vector space structure.
It is naturally endowed with an action of the algebraic group ∏i∈ Q0 GL(d(''i'')) by simultaneous base change. Such action induces one on the ring of functions. The ones which are invariants up to a character of the group are called semi-invariants. They form a ring whose structure reflects representation-theoretical properties of the quiver.
== Definitions ==

Let Q = (Q0,Q1,''s'',''t'') be a quiver. Consider a dimension vector d, that is an element in ℕQ0. The set of d-dimensional representations is given by
: \operatorname(Q,\mathbf):=\(i)\}
Once fixed bases for each vector space ''V''''i'' this can be identified with the vector space
: \bigoplus_ \operatorname_k(k^, k^)
Such affine variety is endowed with an action of the algebraic group GL(d) := ∏''i''∈ Q0 GL(d(''i'')) by simultaneous base change on each vertex:
:
\begin
GL(\mathbf) \times \operatorname(Q,\mathbf) & \longrightarrow & \operatorname(Q,\mathbf)\\
\Big((g_i), (V_i, V(\alpha))\Big) & \longmapsto & (V_i,g_\cdot V(\alpha)\cdot g_^ )
\end

By definition two modules ''M'',''N'' ∈ Rep(Q,d) are isomorphic if and only if their GL(d)-orbits coincide.
We have an induces action on the coordinate ring ''k''() by defining:
:
\begin
GL(\mathbf) \times k() & \longrightarrow & k()\\
(g, f) & \longmapsto & g\cdot f(-):=f(g^. -)
\end


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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