Editing Appendix/Mathematics/ToroidalFunctions (section)

==CT99 Coordinates==

<font color="red"><b>Eureka!</b></font> Via his dogged efforts and an extraordinarily in-depth investigation of this problem, [[Appendix/Ramblings/CCGF#Compact_Cylindrical_Green_Function_.28CCGF.29|in 1999 Howard S. Cohl discovered]] that, in cylindrical coordinates, the relevant Green's function can be written in a much more compact and much more practical form. Specifically, he realized that,
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<math>~ \frac{1}{|\vec{x} - \vec{x}^{~'}|}</math>
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<math>~=</math>
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<math>~ 
\frac{1}{\pi \sqrt{\varpi \varpi^'}} \sum_{m=-\infty}^{\infty} e^{im(\phi - \phi^')}Q_{m- 1 / 2}(\chi) \, ,
</math>
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where,<br />
<div align="center"><math>~\chi \equiv \frac{\varpi^2 + (\varpi^')^2 + (z - z^')^2}{2\varpi \varpi^'} \, ,</math><br /><br />
[http://adsabs.harvard.edu/abs/1999ApJ...527...86C H. S. Cohl &amp; J. E. Tohline (1999)], p. 88, Eqs. (15) &amp; (16)<br />
See also: [http://adsabs.harvard.edu/abs/2007AmJPh..75..724S Selvaggi, Salon &amp; Chari (2007)] &sect;II, eq. (5)<br />
and the [https://dlmf.nist.gov/14.19#ii DLMF's definition of Toroidal Functions], <math>~Q_{m - 1 / 2}^{0}</math>
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and, <math>~Q_{m - 1 / 2}</math> is the zero-order, half-(odd)integer degree, Llegendre function of the second kind &#8212; also referred to as a ''toroidal'' function of zeroth order; see [[#Toroidal_Functions|additional details, below]].  Hence, anywhere along the boundary of our cylindrical-coordinate mesh, a valid expression for the gravitational potential is,

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<math>~ \Phi_B(\varpi,\phi,z)</math>
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<math>~=</math>
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<math>~ 
-G \int \frac{1}{|\vec{x}^{~'} - \vec{x}|} ~\rho(\vec{x}^{~'}) d^3x^' 
</math>
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&nbsp;
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<math>~=</math>
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<math>~ 
-G \int \biggl\{ 
\frac{1}{\pi \sqrt{\varpi \varpi^'}} \sum_{m=-\infty}^{\infty} e^{im(\phi - \phi^')}Q_{m- 1 / 2}(\chi)
\biggr\}~
\rho(\varpi^',\phi^',z^') \varpi^' d\varpi^' d\phi^' dz^' 
</math>
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</tr>

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&nbsp;
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<math>~=</math>
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<math>~ 
-\frac{G}{\pi \sqrt{\varpi}} \int \rho(\varpi^',\phi^',z^') \sqrt{\varpi^'} d\varpi^' d\phi^' dz^'  
\sum_{m=0}^{\infty} \epsilon_m \cos[m(\phi - \phi^')] Q_{m- 1 / 2}(\chi) \, ,
</math>
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[http://adsabs.harvard.edu/abs/1999ApJ...527...86C H. S. Cohl &amp; J. E. Tohline (1999)], p. 88, Eq. (18)
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where, <math>~\epsilon_m</math> is the Neumann factor, that is, <math>~\epsilon_0=1</math> and <math>~\epsilon_m = 2</math> for <math>~m\ge 1</math>.  Following this discovery, most of my research group's 3D numerical modeling of self-gravitating fluids has been conducted using ''Toroidal functions'' instead of ''Spherical Harmonics'' to evaluate the boundary potential on our cylindrical-coordinate meshes.

In our [[2DStructure/ToroidalCoordinates#Statement_of_the_Problem|accompanying statement of this problem]], we have written,
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<math>~\Phi(R_*,Z_*)</math>
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<math>~=</math>
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<math>~- \frac{2G}{R_*^{1 / 2}} \int\int \varpi^{1 / 2} \mu K(\mu) \rho(\varpi, Z) d\varpi dZ \, ,</math>
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where, <math>~K(\mu)</math> is the complete elliptic integral of the first kind, and,
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<math>~\mu^2</math>
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<math>~=</math>
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<math>~
\biggl[\frac{4R_*\varpi}{(R_* + \varpi)^2 + (Z_* - Z)^2} \biggr] \, .
</math>
  </td>
</tr>
</table>
</div>