The Yang-Mills Equations
From now on we deal with a certain orbit space M. This “moduli space” is the set of solutions to the self-dual Yang-Mills equations, but divided out by a natural equivalence. The self-dual Yang-Mills equations are an elliptic (except along orbits of the group of gauge transformations) system of partial differential equations for a connection on a vector bundle over a smooth 4-manifold. The group of gauge transformations is the group of natural equivalences in the vector bundle, hence the natural equivalence in the problem, and so it acts on the space of solutions to the self-dual Yang-Mills equations.
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