The $\mathrm{sd}$ function is defined by $$\mathrm{sd}(u)=\dfrac{\mathrm{sn}(u)}{\mathrm{dn}(u)},$$ where $\mathrm{sn}$ is the Jacobi sn function and $\mathrm{dn}$ is the Jacobi dn function.
Special functions by Leon Hall
