= Solution
In the radial-residual convention $\delta r=a_4\cos4\theta$, \b[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 https://arxiv.org/abs/astro-ph/9610055[Central structure measurements], rather than a universal mass rule, and brightness-residual Fourier coefficients must first be converted to this radial convention.
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