Determine the concentration gradient outside the tumor as a function of r, in the absence of convection.

4) Consider a spherical tumor, of radius R_T, releasing the angiogenic factor VEGF into the surrounding tissue. Assume the tumor is an infinite source of VEGF at the concentration C_T, and this drops to 0 as r approaches infinity. Determine the concentration gradient outside the tumor as a function of r, in the absence of convection.

5) Consider the tumor of the problem (A), but with a known flux at the tumor perimeter of J_T.