$M_{\nu }^2\left(x\right)\sim \frac{2}{\pi x}(1+\frac{1}{2}\frac{\mu -1}{{\left(2x\right)}^2}+\frac{1\cdot 3}{2\cdot 4}\frac{(\mu -1)(\mu -9)}{{\left(2x\right)}^4}+\frac{1\cdot 3\cdot 5}{2\cdot 4\cdot 6}\frac{(\mu -1)(\mu -9)(\mu -25)}{{\left(2x\right)}^6}+\cdots )$