The classical disturbing function R is not suitable to most of the asteroids because in the integration of R, the very small divisor (k_1 n + k_2 n'), in the case -k_1/k_2 ~ n'/n, gives origin to a very large period inequality. In order to avoid such a difficulty, we have solved the differential equation of the Hansen-Bohlin method, by means of trigonometrical series using as independent variable ζ= A ∫ A_0 ^-3 (t) dt, instead of the eccentric anomaly. We used an analytical method and an application is made in the particular case of asteroid Thule for whicn n'/n ~ 3/4.