a = - n - \frac{2}{π} \sqrt{z n} - \frac{2 z}{π^{2}} + \frac{1}{2} b + \frac{1}{4} + \frac{z^{2} (\frac{1}{3} - 4 π^{-2}) + z - {(b - 1)}^{2} + \frac{1}{4}}{4 π \sqrt{z n}} + 𝒪 (\frac{1}{n})