Reduced covariance coupling kernels for ℓ = 200 and ℓ’′= ℓ + Δ, with Δ = [0, 10, 50] shown in the three different columns. All kernels in one column share the same color map. All of the matrices are shown as a function of ℓ₁ and ℓ₂ and are centered on = (ℓ + ℓ′)/2. Each of the displayed kernels is properly normalized according to Eq. (22). The plots are restricted to the elements that have a significant value. Top row: approximated INKA kernels and the positions at which the delta distributions of the NKA (circles) and FRI (crosses) approximation peak. Bottom row: exact kernels. The comparison between the two highlights how much of the structure of the exact kernel is missed by the different approximations.