 Solution

In the radial-residual convention $\delta r=a_4\cos 4\theta$, boxy isophotes have $a_4<0$ and disky isophotes have $a_4>0$, since negative $a_4$ extends the diagonal directions and positive $a_4$ extends the fitted axes. Massive elliptical galaxies tend to have slowly rotating, anisotropic stellar structures associated with dry galaxy mergers, whereas rotating embedded galactic disks, often associated with dissipation in gas-rich galaxy mergers, favour disky structure in lower-mass elliptical galaxies. These are population trends, as in Central structure measurements, rather than a universal mass rule, and brightness-residual Fourier coefficients must first be converted to this radial convention.