\[B_{0}(\xi)=-\frac{1}{2}\left(\frac{1-4\alpha^{2}}{8}+\xi\left(\frac{1-x}{x}%
\right)^{\frac{1}{2}}\left(\frac{4\alpha^{2}-1}{8}+\frac{1}{4}\frac{x}{1-x}+%
\frac{5}{24}\left(\frac{x}{1-x}\right)^{2}\right)\right).\]