Improvements to the gravity derivative descriptions in the TeX document.

theory/Multipoles/potential_derivatives.tex

@@ -18,8 +18,10 @@ by constructing derivatives of the truncated potentials:
   \chi^{(4)}(r, r_s) &= \frac{16}{r_s^4} \left(48\alpha(x)^5 - 120\alpha(x)^4 + 100\alpha(x)^3 -30 \alpha(x)^2 + 2\alpha(x)\right) \nonumber \\ 
   \chi^{(5)}(r, r_s) &= \frac{32}{r_s^5} \left(240\alpha(x)^6 - 720\alpha(x)^5 + 780\alpha(x)^4 - 360\alpha(x)^3 + 62\alpha(x)^2 - 2\alpha(x) \right) \nonumber
 \end{align}
-We can now construct common quantities that appear in derivatives of
-multiple orders:
+In the Newtonian limit ($r_s\rightarrow\infty$) the first expression
+reduces to $\chi(r,r_s) = 1$ whilst all higher-order derivatives
+vanish. We can now construct common quantities that appear in
+derivatives of multiple orders:
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \begin{align}