\[B=2^{-2\sigma}\frac{{\Gamma}^{2}\left(\tfrac{1}{2}(1-\sigma)\right)\Gamma\left%
(\tfrac{1}{2}(1+\sigma)+\nu\right)}{{\Gamma}^{2}\left(\tfrac{1}{2}(1+\sigma)%
\right)\Gamma\left(\tfrac{1}{2}(1-\sigma)+\nu\right)}.\]