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A Quantum Kirwan Map: Bubbling and Fredholm Theory for ... - KIAS

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2.5. COMPACTNESS MODULO BUBBLING AND GAUGE FOR RESCALED VORTICES 31Note also that the action of Isom + (C) on the set of vortex classes of positiveenergy is not always free, as the next example shows. ✷34. Example. Consider the action of G := S 1 ⊆ C on M := C by multiplication.Let d ∈ N 0 be an integer. By Proposition 26 there exists a unique finiteenergy vortex class W over C such that{ d, if z = 0,deg W (z) =0, otherwise.For every rotation R ∈ SO(2) we havedeg R ∗ W = R ∗ deg W = deg W ,where SO(2) acts in a natural way on Sym d (C). Thus the action of Isom + (C) onthe set of vortex classes of positive energy is not free. ✷2.5. Compactness modulo bubbling <strong>and</strong> gauge <strong>for</strong> rescaled vorticesIn this section we <strong>for</strong>mulate <strong>and</strong> prove a crucial ingredient (Proposition 37) ofthe proof of Theorem 3, which states the following. Consider a sequence of rescaledvortices over C with image in a fixed compact subset of M <strong>and</strong> uni<strong>for</strong>mly boundedenergies. We assume that (M,ω) is aspherical. Then there exists a subsequencethat away from finitely many bubbling points <strong>and</strong> up to regauging, converges toa rescaled vortex over C. If the rescalings converge to ∞, then the limit objectcorresponds to a ¯J-holomorphic sphere in M.The proof of this result is based on compactness <strong>for</strong> rescaled vortices over thepunctured plane with uni<strong>for</strong>mly bounded energy densities (Proposition 38 below).It also uses the fact that at each bubbling point at least the energy E min > 0 islost, which is the minimal energy of a vortex over C or pseudo-holomorphic spherein M. This is the content of Proposition 40 below, which is proved by a hardrescaling argument, using Proposition 38 <strong>and</strong> Hofer’s lemma. Another ingredientof the proof of Proposition 37 is Lemma 42 below, which says that the energydensities of a convergent sequence of rescaled vortices converge to the density ofthe limit.In order to explain the main result of this section, let M,ω,G,g, 〈·, ·〉 g ,µ,J,Σ,j, <strong>and</strong> ω Σ be as in Chapter 1. We fix a triple w = (P,A,u) ∈ ˜B Σ (defined as in(1.7)). Recall the definition (1.11) of the energy density ew ωΣ,j = e w .35. Remark. This density has the following trans<strong>for</strong>mation property: Let Σ ′be another surface, <strong>and</strong> ϕ : Σ ′ → Σ a smooth immersion. Consider the pullbackϕ ∗ w := ( ϕ ∗ P,Φ ∗ A,u ◦ Φ ) ,where the bundle isomorphism Φ : ϕ ∗ P → P is defined by Φ(z,p) := p. A straight<strong>for</strong>wardcalculation shows that(2.26) e ϕ∗ (ω Σ,j)ϕ ∗ w= e ωΣ,jw ◦ ϕ.Note also that w is a vortex with respect to (ω Σ ,j) if <strong>and</strong> only if ϕ ∗ w is a vortexwith respect to ϕ ∗ (ω Σ ,j). ✷36. Remark. If w is a vortex (with respect to (ω Σ ,j)) then(2.27) e ωΣ,jw = |∂ J,A u| 2 + |µ ◦ u| 2 ,

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