\[K(z,\zeta)=k\operatorname{Ai}\left(\frac{z+\zeta}{2}\right)+\frac{k^{2}}{4}\*%
\int_{z}^{\infty}\!\!\!\int_{z}^{\infty}K(z,s)\operatorname{Ai}\left(\frac{s+t%
}{2}\right)\operatorname{Ai}\left(\frac{t+\zeta}{2}\right)\,\mathrm{d}s\,%
\mathrm{d}t,\]