Model CO2 diffusing across the wall of a cylindrical alveolar blood vessel

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TL;DR Summary: Solve heat equation in a disc using fourier transforms

Carbon dioxide dissolves in the blood plasma but is not absorbed by red blood cells. As the blood returns to an alveolus, assume that it is well-mixed so that the concentration of dissolved CO2 is uniform across a cylindrical alveolar blood vessel. Model the diffusive transport of CO2 from an infinitely long cylinder of radius a containing plasma to the alveolus wall, i.e. use dc/dt = D d^2 c/dr^2 where r is the radial component, t is time, D is a diffusion constant and c(r,t) is the concentration of CO2. This is subject to initial and boundary conditions c(r,0)=c_0>1 and c(a,t)=1. Find c(r,t) using Fourier transforms.

I was given the hint to find the steady state solution first and then subtract it from the full solution to solve with FTs for homogeneous BCs. I'm stuck though, I feel like there isn't enough boundary conditions. Any insight would be great!
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
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