Unstable mode for the truncated ν_t = 4 Mestel disc recovered via the matrix method presented in Sect. 3. Only positive contour levels are shown and are spaced linearly between 10% and 90% of the maximum norm. The radii associated with the resonances ILR, COR and OLR have been represented, as given by ω₀ = m·Ω(R_m), where the intrinsic frequencies Ω(R) = (Ω_φ(R),κ(R)) have to be computed within the epicyclic approximation. For a Mestel disc, they are given by Ω_φ(R) = V₀/R and .