d2u+ m2u = 0,
dθ2

(19)

the solution of which is

au = sin m (θ − α).

(20)

The orbit has therefore two asymptotes, inclined at an angle π/m. In the latter case the differential equation is of the form

d2u= m2u,
dθ2

(21)

so that

u = Aemθ + Be−mθ