 Instanton moduli space

For a fixed principal bundle, orientation and instanton sign, quotient the Yang-Mills instanton solutions by gauge equivalence of principal connections. Boundary or framing conditions determine the gauge group $\mathcal{G}$ on a noncompact base. At an anti-self-dual solution, infinitesimal deformations obey $P_{+}{D}_{A}a=0$ modulo $a\mapsto a+D_A\epsilon$. Combining this with $D_A^{\ast }a=0$ gives an elliptic deformation operator. This description does not assert that every quotient is a smooth manifold: reducible connections can have nontrivial stabilizers.