Editing SSC/Perturbations (section)

===Review Article by Christy (1966)===
In the review by [http://adsabs.harvard.edu/abs/1966ARA%26A...4..353C R. F. Christy (1966)] titled, ''Pulsation Theory,'' this key 2<sup>nd</sup>-order ODE appears as equation (5) in the form shown by the following boxed-in (slightly edited) excerpt:

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''Pulsation Equation'' extracted<sup>&dagger;</sup> from [http://adsabs.harvard.edu/abs/1966ARA%26A...4..353C R. F. Christy (1966)]<p></p>
"''Pulsation Theory''"<p></p>
Annual Review of Astronomy &amp; Astrophysics<sup>&reg;</sup>, vol. 4, pp. 353 - 392 &copy; [http://www.annualreviews.org/ Annual Reviews]
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[[File:Cox1974Eq5.png|600px|center|Christy (1966)]]
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<sup>&dagger;</sup>Mathematical expressions displayed here, as a single digital image, with presentation order &amp; layout modified from the original publication.<p></p>
<sup>&reg;</sup>Copyright 2015 Annual Reviews.  All rights reserved.  The Annual Reviews logo, and other Annual Reviews products referenced herein are either registered trademarks or trademarks of Annual Reviews.  All other marks are the property of their respective owner and/or licensor.
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Correspondence with our derived equation becomes clear by appreciating that Christy's <math>~x \equiv r_0/R</math> and that,
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<math>GM(x) = g_0 r_0^2 \, ,</math>
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hence, 
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<math>V(x) \rightarrow \frac{g_0 \rho_0 r_0}{P_0} \, .</math>
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Making these substitutions and multiplying Christy's equation through by <math>~R^{-2}</math> gives,

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<math>~
\frac{d^2\xi}{dr_0^2} + \biggl[ \frac{4}{r_0} - \frac{g_0 \rho_0}{P_0} \biggr]\frac{d\xi}{dr_0}
+ \frac{\rho_0}{\gamma_g P_0 } \biggl[(4-3\gamma_g)\frac{g_0}{r_0} +  \omega^2 \biggr] \xi
</math>
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<math>~=</math>
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<math>~
0\, .
</math>
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