Editing PGE/PoissonOrigin (section)

==Step 1==
In the astrophysics literature, it is customary to adopt the following definition of the,
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<font color="#770000">'''Scalar Gravitational Potential'''</font>
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<math>\Phi(\vec{x})</math>
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<math>\equiv</math>
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<math>-G \int \frac{\rho(\vec{x}^{~'})}{|\vec{x}^{~'} - \vec{x}|} d^3x^' \, .</math>
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[<b>[[Appendix/References#BT87|<font color="red">BT87</font>]]</b>], p. 31, Eq. (2-3)<br />
[<b>[[Appendix/References#EFE|<font color="red">EFE</font>]]</b>], &sect;10, p. 17, Eq. (11)<br />
[<b>[[Appendix/References#T78|<font color="red">T78</font>]]</b>], &sect;4.2, p. 77, Eq. (12)
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(Note: &nbsp; As we have detailed in a [[VE#Setting_the_Stage|separate discussion]], throughout [<b>[[Appendix/References#EFE|<font color="red">EFE</font>]]</b>] Chandrasekhar adopts a ''different sign convention'' as well as a different variable name to represent the gravitational potential.)  Recognizing that the gradient of the function, <math>~|\vec{x}^{~'} - \vec{x}|^{-1}</math>, with respect to {{ Template:Math/VAR_PositionVector01 }} is,
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<math>\nabla_x \biggl[ \frac{1}{|\vec{x}^{~'} - \vec{x}|} \biggr]</math>
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<math>=</math>
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<math>
\frac{\vec{x}^{~'} - \vec{x}}{|\vec{x}^{~'} - \vec{x}|^3} \, ,
</math>
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[<b>[[Appendix/References#BT87|<font color="red">BT87</font>]]</b>], p. 31, Eq. (2-4)
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and given that, in the above expression for the gravitational acceleration, the integration is taken over the volume that is identified by the ''primed'' <math>(\vec{x}~{'})</math>, rather than the unprimed <math>(\vec{x})</math>, coordinate system, <font color="#007700">we find that we may write</font> the gravitational acceleration as,
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<math>\vec{a}(\vec{x})</math>
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<math>=</math>
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<math>\int G\rho(\vec{x}^{~'})  \nabla_x \biggl[ \frac{1}{|\vec{x}^{~'} - \vec{x}|} \biggr]d^3 x'
</math>
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&nbsp;
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<math>=</math>
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<math>\nabla_x \biggl\{ G \int \biggl[ \frac{\rho(\vec{x}^{~'}) }{|\vec{x}^{~'} - \vec{x}|} \biggr]d^3 x'\biggr\}</math>
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&nbsp;
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<math>=</math>
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<math>-\nabla_x \Phi \, .</math>
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[<b>[[Appendix/References#BT87|<font color="red">BT87</font>]]</b>], p. 31, Eq. (2-5)
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