Kelvin kei
From specialfunctionswiki
The $\mathrm{bei}_{\nu}$ function is defined as $$\mathrm{ber}(z)=\mathrm{Im} \hspace{2pt} K_{\nu} \left( x e^{\frac{\pi i}{4}} \right),$$ where $\mathrm{Im}$ denotes the imaginary part of a complex number and $K_{\nu}$ denotes the modified Bessel $K_{\nu}$.
