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I am trying to solve something like Fick's Law using NDSolve: $$\frac{\partial \varphi}{\partial t}=\frac{\partial^2 \varphi}{\partial r^2}+F(r,t)$$ Subject to a boundary condition which looks something like: $$\varphi (R,t)=\varphi_0-\int_0^R \varphi(r,t)dr$$ Which is an attempt to simulate the case where whatever is diffusing into the medium of interest is limited in quantity. I assume simply plugging a recursive boundary condition is going to go poorly (Edit: it did go poorly), is there any option here other than writing my own solver?

In specific one particular case could look like: $$\frac{\partial \varphi}{\partial t}=\frac{\partial^2 \varphi}{\partial r^2}+\frac{1}{r^2}\frac{\partial\varphi}{\partial r}(2r+r^3/3-\int\varphi r^2dr)-\varphi$$ In this case $\varphi_0=1$ and the initial condition is $\varphi(0\leq r<R,0)=0$, where $R=1$. The problem should have spherical symmetry so a bound at $r=0$ may result in trouble.

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    – Dunlop
    Sep 22, 2020 at 3:32
  • $\begingroup$ The method here may be helpful. $\endgroup$
    – bbgodfrey
    Sep 22, 2020 at 17:00
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    $\begingroup$ Readers are more likely to respond, if you provide all the necessary information to obtain a numerical solution, including the definitions of F, the values of R and phi0, and the initial condition and other boundary condition. $\endgroup$
    – bbgodfrey
    Sep 22, 2020 at 18:44
  • $\begingroup$ As it turns out the method of lines was a nudge in the right direction, I found a few more posts on the topic of integro-differential equations and their boundary conditions at it looks like there are a few other avenues to try (possibly replacing $\varphi$ with its x-partial-derivative to eliminate the integral terms?). $\endgroup$ Sep 23, 2020 at 14:07
  • $\begingroup$ @UranylTrioxide What was the outcome of your effort? $\endgroup$
    – bbgodfrey
    Nov 16, 2020 at 4:16

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