= Pointwise alpha-prescription obstruction in a radiation-supported layer
In the exact constant-<opacity>, pure-<radiation pressure> diffusion model, <dynamic viscosity> is height-independent. Imposing an additional exact pointwise relation $\mu\propto p(z)$ forces constant pressure, incompatible with nonzero vertical <hydrostatic equilibrium> away from the midplane. The characteristic-pressure <alpha disk> prescription can still be used for order-of-magnitude scalings; it is not an exact simultaneous vertical solution.
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