 In Ω-background, we can integrate out all the non-zero modes, so the path integral with the boundary condition formula_19 at formula_20 
can be expressed as a sum over instanton number of the products and ratios of fermionic and bosonic determinants, producing the so-called Nekrasov partition function.In the limit where formula_12, formula_17 approach 0, this sum is dominated by a unique saddle point.On the other hand, when formula_12, formula_17 approach 0,
holds.