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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 : Once fixed bases for each vector space ''V''''i'' this can be identified with the vector space : 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: : 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: : 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Semi-invariant of a quiver」の詳細全文を読む スポンサード リンク
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