Looking at transmission shows that this last expression boils down to the expected Breit–Wigner (Lorentzian) formula, as 1 − − σ σ ( q = 0 ) ≈ ≈ ( Γ Γ r e s / 2 ) 2 ( Γ Γ r e s / 2 ) 2 + ( E − − E r e s ) 2 = f ( E ; E r e s , Γ Γ r e s / 2 , 1 ) , the three parameters Lorentzian function (note that it is not a density function and does not integrate to 1, as its amplitude I is 1 and not 2 / π π Γ Γ r e s ).