From these conditions we fix completely the two parameters to α = − 1 and β = 0 so that the mass function (in the y ≤ 0 region ) becomes: The most general condition that we can impose on the term multiplying to 1 / [ y (1 − y )] in order that the equation be that of the hypergeometric function is that it be equal to a constant γ . Therefore we get three equations: From Eq. (16a), it is possible to solve for µ while λ is found summing Eq. (16b) and Eq. (16c). We obtain finally: having defined ν = ik/a where k 2 = E 2 − m 2 $_{0}$. Our Eq. (13) becomes the differential equation of the hypergeometric function and the general solution is (with D$_{1}$ and D$_{2}$ constants): 7 f ( y ) = D$_{1 2}$F$_{1}$ ( µ − ν − λ, µ + ν − λ ; 2 µ ; y )+ D$_{2}$ y 1 − 2 µ $_{2}$F$_{1}$ (1 − µ − ν − λ, 1 − µ + ν − λ ; 2 − 2 µ ; y ) . (19) y (1 − y ) d $^{2}$f ( y ) dy 2 + [2 µ − (1 + 2 µ − 2 λ ) y ] d f ( y ) dy − ( µ − λ − ν )( µ − λ + ν ) f = 0 , (18) γ = ν 2 − µ 2 − λ ( λ + 1) (17c) λ = i$^{√}$ W 2 − m 2 0 a 2 (17a) µ = − i ( E − W ) a (17b) µ 2 + λ (1 + λ ) + E 2 − m 2 0 a 2 = − γ (16c) − 2 E 2 a 2 + 2 EW a 2 − i W − E a − 2 µ 2 − ( λ − µ ) = γ (16b) µ 2 + E 2 a 2 + W 2 a 2 − 2 EW a 2 − µ − i a ( E − W ) = 0 (16a) m ( y ) = m$_{0}$ y y − 1 (15)