Kelvin bei

The $\mathrm{bei}_{\nu}$ function is defined as $$\mathrm{bei}_{\nu}(z)=\mathrm{Im} \hspace{2pt} J_{\nu} \left( z e^{\frac{3\pi i}{4}} \right),$$ where $\mathrm{Im}$ denotes the imaginary part of a complex number and $J_{\nu}$ denotes the Bessel function of the first kind.