Frequency kernels versus ratio kernels of the model 1.3MoAGSS from Sect. 3.1. For both panels, we show the kernel amplitude as a function of the position, and for the structural variable pair (c², ρ). Upper panel: low-order (l = 0, n = 11) and high-order (l = 0, n = 26) kernels for the individual frequencies, respectively in orange and blue. Lower panel: low-order (n = 11) and high-order (n = 26) kernels for the frequency separation ratios, respectively in orange and blue.