Boundary ambiguity of a flat disk rotation curve
= Boundary ambiguity of a flat disk rotation curve
The midplane <galaxy rotation curve> fixes the radial derivative of the <Newtonian gravitational potential>, not every exterior boundary condition. Adding $a|z|$ leaves that radial derivative unchanged but adds a uniform sheet of <surface density of a disk> $a/(2\pi G)$. The pure scale-free <Mestel disk> excludes such extra sources.