# Kochin Function

The Kochin function $\mathbf{H}(\tau)$ is given by
$\mathbf{H}(\tau)=\iint_{\Gamma_s}\left( -\frac{\delta\phi}{\delta n} + \phi\frac{\delta}{\delta n}\right) e^{kz}e^{ik(x\cos\tau +y\sin\tau)}\mathrm{d}S,$
where $\delta/\delta n$ is the inward normal derivative, $\phi$ is the velocity potential and $\Gamma_s$ is the wetted body surface.