Earth gravitational potential

Question

In the Cook's "Perturbations of satellite orbits by tesseral harmonics" i've found the following formula(13, page 799) for potential: $$U = \frac{\mu}{r}\left\{1-\sum\limits_{n=2}^{\infty}\left(\frac{R}{r}\right)^n\left[J_nP_n(\sin\phi)-\sum_{s=1}^{n}J_{n,s}P_n^s\sin\phi\cos{s(\lambda - \lambda_{n,s})}\right] \right\}$$ What values are taken as $\lambda_{n,s}$ and $J_{n,s}$? There were some words about it in the book, but not the exact values.