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 Admin P: 21,827 Certainly the problem can be solved numerically (FD or FE), and I believe analytically, but I'd have to dig back in my archives for that. one could write \nu_1 and \nu_2 in thex LaTeX expressions before \Sigma. I think this is how the equations are supposed to look: $$-{D_1}\frac{{d^2}\phi_1}{dx^2}\,+\,\Sigma_{R1}\phi_1\,=\,\frac{1}{k}({\n u_1}{\Sigma_{f1}\phi_1}\,+\,{\nu_2}{\Sigma_{f2}\phi_2})$$ $$-{D_2}\frac{{d^2}\phi_2}{dx^2}\,+\,\Sigma_{a2}\phi_2\,=\,{\Sigma_{s12}}\ phi_1$$ Just looking these, one could collect coefficents and rewrite the equations as: $\phi_1$'' + A $\phi_1$ = B $\phi_2$ $\phi_2$'' + C $\phi_2$ = D $\phi_1$