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Given a concrete category C, with objects denoted Obj(C), and an equivalence relation ~ on Obj(C) given by morphisms in C. The moduli set for Obj(C) is the set of equivalence classes with respect to ~; denoted Iso(C). When Iso(C) is an object in the category Top, then the moduli set is called a moduli space.

7 votes

Hitchin fibration and Springer resolution

I will try to answer the first question only. As in the remarks, the canonical reference is Beauville, Narasimhan, Ramanan, Spectral curves and the generalised theta divisor. J. Reine Angew. Math. …
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How do we get the quotient $Ext^1(N,M)/Hom(N,M)$?

The action is trivial, as you wrote. Generally if $G/S$ acts trivially on $X/S$ and $G$ is abelian then any object in the quotient stack $[X/G]=B_X G$ has an automorphism group canonically isomorphic …
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