User:Tohline/2DStructure/ToroidalGreenFunction

Using Toroidal Coordinates to Determine the Gravitational Potential

NOTE:   An earlier version of this chapter has been shifted to our "Ramblings" Appendix.

Here we build upon our accompanying review of the types of numerical techniques that various astrophysics research groups have developed to solve for the Newtonian gravitational potential, <math>~\Phi(\vec{x})</math>, given a specified, three-dimensional mass distribution, <math>~\rho(\vec{x})</math>. Our focus is on the use of toroidal coordinates to solve the integral formulation of the Poisson equation, namely,

<math>~ \Phi(\vec{x})</math>

<math>~=</math>

<math>~ -G \iiint \frac{\rho(\vec{x}^{~'})}{|\vec{x}^{~'} - \vec{x}|} d^3x^' \, .</math>

For the most part, we will adopt the notation used by C.-Y. Wong (1973, Annals of Physics, 77, 279); in an accompanying discussion, we review additional results from this insightful 1973 paper, as well as a paper of his that was published the following year in The Astrophysical Journal, namely, Wong (1974).

Basic Elements of a Toroidal Coordinate System

Given the meridional-plane coordinate location of a toroidal-coordinate system's axisymmetric anchor ring, <math>~(\varpi,z) = (a,Z_0)</math>, the relationship between toroidal coordinates <math>~(\eta,\theta,\psi) </math>and Cartesian coordinates <math>~(x, y, z)</math> is,

<math>~x</math>	<math>~=</math>	<math>~\frac{a \sinh\eta \cos\psi}{(\cosh\eta - \cos\theta)} \, ,</math>
<math>~y</math>	<math>~=</math>	<math>~\frac{a \sinh\eta \sin\psi}{(\cosh\eta - \cos\theta)} \, ,</math>
<math>~z - Z_0</math>	<math>~=</math>	<math>~\frac{a \sin\theta}{(\cosh\eta - \cos\theta)} \, .</math>

This set of coordinate relations appears as equations 2.1 - 2.3 in Wong (1973). This set of relations may also be found, for example, on p. 1301 within eq. (10.3.75) of [MF53]; in §14.19 of NIST's Digital Library of Mathematical Functions; or even within Wikipedia. (In most cases the implicit assumption is that <math>~Z_0 = 0</math>.)

Mapping the other direction [see equations 2.13 - 2.15 of Wong (1973) ], we have,

<math>~\eta</math>	<math>~=</math>	<math>~\ln\biggl(\frac{r_1}{r_2} \biggr) \, ,</math>
<math>~\cos\theta</math>	<math>~=</math>	<math>~\frac{(r_1^2 + r_2^2 - 4a^2)}{2r_1 r_2} \, ,</math>
<math>~\tan\psi</math>	<math>~=</math>	<math>~\frac{y}{x} \, ,</math>

where,

<math>~r_1^2 </math>	<math>~\equiv</math>	<math>~[(x^2 + y^2)^{1 / 2} + a]^2 + (z-Z_0)^2 \, ,</math>
<math>~r_2^2 </math>	<math>~\equiv</math>	<math>~[(x^2 + y^2)^{1 / 2} - a]^2 + (z-Z_0)^2 \, ,</math>

and <math>~\theta</math> has the same sign as <math>~(z-Z_0)</math>.

According to p. 1301, eq. (10.3.75) of [MF53] — or, for example, as found in Wikipedia — in toroidal coordinates the differential volume element is,

<math>~d^3x</math>

<math>~=</math>

<math>~h_\eta h_\theta h_\psi d\eta d\theta d\psi</math>

<math>~=</math>

<math>~\biggl[ \frac{a^3 \sinh\eta}{(\cosh\eta - \cos\theta)^3} \biggr] d\eta~ d\theta~ d\psi \, .</math>

Selected Toroidal Function Relationships

Here, we draw from the set of toroidal function relationships that have been identified as "key equations" in our accompanying Equations appendix.

Beginning with the identified "Key Equation", <math>~Q_{n-1 / 2}^m (\lambda)</math> <math>~=</math> <math>~(-1)^n \frac{\pi^{3/2}}{\sqrt{2}~ \Gamma(n-m+1 / 2)} (x^2-1)^{1 / 4} P_{m-1 / 2}^n(x) \, , </math> Gil, Segura, & Temme (2000):  eq. (8) where:     <math>~\lambda \equiv x/\sqrt{x^2-1}</math> we'll identify <math>~x</math> with <math>~\cosh\eta</math> — in which case we have <math>~\lambda = \coth\eta</math> — and switch the index notations, <math>~n \leftrightarrow m</math>. This gives, <math>~Q_{m-1 / 2}^n (\coth\eta)</math> <math>~=</math> <math>~(-1)^m \frac{\pi^{3/2}}{\sqrt{2} ~\Gamma(m-n+\frac{1}{2})} (\sinh\eta)^{1 / 2} P_{n-1 / 2}^m(\cosh\eta) \, . </math> Drawing upon the Euler reflection formula for gamma functions, namely, <math>~ \Gamma(z) ~\Gamma(1-z) </math> <math>~=</math> <math>~ \frac{\pi}{\sin(\pi z)} </math> <math>~\biggl|</math> for example, if <math>~z \rightarrow (m-n + \tfrac{1}{2})</math> <math>~\Rightarrow ~~~\Gamma(m-n+\tfrac{1}{2})~\Gamma(n-m+\tfrac{1}{2})</math> <math>~=</math> <math>~\pi \biggl\{\sin\biggl[ \frac{\pi}{2} + \pi(m-n) \biggr] \biggr\}^{-1}</math>   <math>~=</math> <math>~\pi (-1)^{m-n} </math> DLMF §5.5(ii) <math>~\biggl|</math> Valid for:    <math>~z \ne0, \pm 1, \pm 2, </math> … <math>~\biggl|</math> where it is understood that <math>~m</math> and <math>~n</math> are each either zero or a positive integer, this toroidal-function relation becomes, <math>~Q_{m-1 / 2}^n (\coth\eta)</math> <math>~=</math> <math>~(-1)^m~ \frac{\pi^{3/2}}{\sqrt{2} } \biggl[ \frac{ \Gamma(n - m +\frac{1}{2}) }{ \pi (-1)^{m-n} } \biggr] (\sinh\eta)^{1 / 2} P_{n-1 / 2}^m(\cosh\eta) </math>     <math>~=</math> <math>~ (-1)^n \sqrt{ \frac{\pi}{2} } ~\Gamma(n - m + \tfrac{1}{2} )(\sinh\eta)^{1 / 2} P_{n-1 / 2}^m(\cosh\eta) \, . </math> ①

Again, beginning with the identified "Key Equation", <math>~Q_{n-1 / 2}^m (\lambda)</math> <math>~=</math> <math>~(-1)^n \frac{\pi^{3/2}}{\sqrt{2}~ \Gamma(n-m+1 / 2)} (x^2-1)^{1 / 4} P_{m-1 / 2}^n(x) \, , </math> Gil, Segura, & Temme (2000):  eq. (8) where:     <math>~\lambda \equiv x/\sqrt{x^2-1}</math> this time, without switching index notations, we'll identify <math>~x</math> with <math>~\coth\eta</math> — in which case we have <math>~\lambda = \cosh\eta</math>. This gives, <math>~Q_{n-1 / 2}^m (\cosh\eta)</math> <math>~=</math> <math>~(-1)^n \frac{\pi^{3/2}}{\sqrt{2} ~\Gamma(n-m+\frac{1}{2})} \biggl( \frac{1}{\sinh\eta} \biggr)^{1 / 2} P_{m-1 / 2}^n(\coth\eta) \, . </math> Drawing upon the same Euler reflection formula for gamma functions, as quoted above, this toroidal function relation can be rewritten as, <math>~Q_{n-1 / 2}^m (\cosh\eta)</math> <math>~=</math> <math>~(-1)^n \frac{\pi^{3/2}}{\sqrt{2} } \biggl[ \frac{\Gamma(m-n+\frac{1}{2})}{\pi (-1)^{m-n}} \biggr] \biggl( \frac{1}{\sinh\eta} \biggr)^{1 / 2} P_{m-1 / 2}^n(\coth\eta) </math>   <math>~=</math> <math>~(-1)^{-m}~ \sqrt{\frac{\pi}{2}} \biggl[ \frac{\Gamma(m-n+\frac{1}{2})}{\sqrt{\sinh\eta}} \biggr] P_{m-1 / 2}^n(\coth\eta) \, . </math> Finally, calling upon the "Key Equation" relation, <math>~ P_\nu^n(z) </math> <math>~=</math> <math>~ \frac{\Gamma(\nu + n + 1)}{\Gamma(\nu - n + 1)} P_\nu^{-n}(z) </math> A. Erdélyi (1953):  Volume I, §3.3.1, p. 140, eq. (7) making the index notation substitution, <math>~\nu \rightarrow (m-\tfrac{1}{2})</math>, and associating <math>~z</math> with <math>~ \coth\eta</math> gives, <math>~P^n_{m-\frac{1}{2}}(\coth\eta)</math> <math>~=</math> <math>~\biggl[ \frac{\Gamma(m+n+\frac{1}{2})}{\Gamma(m-n+\frac{1}{2})} \biggr]P^{-n}_{m-\frac{1}{2}}(\coth\eta) \, .</math> As a result, we can write, <math>~Q_{n-1 / 2}^m (\cosh\eta)</math> <math>~=</math> <math>~(-1)^{-m}~ \sqrt{\frac{\pi}{2}} \biggl[ \frac{\Gamma(m+n+\frac{1}{2})}{\sqrt{\sinh\eta}} \biggr] P_{m-1 / 2}^{-n}(\coth\eta) </math>   <math>~\Rightarrow ~~~ P_{m-1 / 2}^{-n}(\coth\eta) </math> <math>~=</math> <math>~(-1)^m~\sqrt{\frac{2}{\pi}} \biggl[ \frac{\sqrt{\sinh\eta}}{\Gamma(m+n+\frac{1}{2})} \biggr] Q_{n-1 / 2}^m (\cosh\eta) \, . </math> ②

Green's Function Expression

As presented by Wong (1973)

Referencing [MF53], Wong (1973) states that, in toroidal coordinates, the Green's function is,

<math>~\frac{1}{|~\vec{x} - {\vec{x}}^{~'} ~|} </math>	<math>~=</math>	<math>~ \frac{1}{\pi a} \biggl[ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^') \biggr]^{1 /2 } \sum\limits^\infty_{m,n=0} (-1)^m \epsilon_m \epsilon_n ~\frac{\Gamma(n-m+\tfrac{1}{2})}{\Gamma(n + m + \tfrac{1}{2})} </math>
		<math>~ \times \cos[m(\psi - \psi^')]\cos[n(\theta - \theta^')] ~\begin{cases}P^m_{n-1 / 2}(\cosh\eta) ~Q^m_{n-1 / 2}(\cosh\eta^') ~~~\eta^' > \eta \\P^m_{n-1 / 2}(\cosh\eta^') ~Q^m_{n-1 / 2}(\cosh\eta)~~~\eta^' < \eta \end{cases}\, , </math>
Wong (1973), p. 293, Eq. (2.53) [see also: J. W. Bates (1997), p. 3685, Eq. (31)] [see also: Cohl, Tohline, Rau, & Srivastava (2000), §6.2, Eq. (48)]

where, <math>~P^m_{n-1 / 2}, Q^m_{n-1 / 2}</math> are Legendre functions of the first and second kind with order <math>~n - \tfrac{1}{2}</math> and degree <math>~m</math> (toroidal harmonics), and <math>~\epsilon_m</math> is the Neumann factor, that is, <math>~\epsilon_0 = 1</math> and <math>~\epsilon_m = 2</math> for all <math>~m \ge 1</math>. This Green's function expression can indeed be found as eq. (10.3.81) on p. 1304 of [MF53], but it should be noted that the MF53 expression differs from Wong's in two respects (see footnote 2 on p. 370 of Cohl et al. (2000) for a proposed explanation): First, the factor, <math>~(-1)^m</math>, appears as <math>~(-i)^m</math> in MF53; and, second, in the term that is composed of a ratio of gamma functions, the denominator appears in MF53 as <math>~\Gamma(n - m + \tfrac{1}{2})</math>, whereas it should be <math>~\Gamma(n + m + \tfrac{1}{2})</math>, as presented here.

Rearranging Terms and Incorporating Special-Function Relations

Let's focus on the situation when <math>~\eta^' > \eta</math>, and begin rearranging or substituting terms.

<math>~\frac{1}{|~\vec{x} - {\vec{x}}^{~'} ~|} </math>	<math>~=</math>	<math>~ \frac{1}{\pi a} \biggl[ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^') \biggr]^{1 /2 } \sum\limits^\infty_{m=0} (-1)^m \epsilon_m \cos[m(\psi - \psi^')] </math>
		<math>~ \times \sum\limits^\infty_{n=0} \epsilon_n \cos[n(\theta - \theta^')] ~\frac{\Gamma(n-m+\tfrac{1}{2})}{\Gamma(n + m + \tfrac{1}{2})} ~P^m_{n-1 / 2}(\cosh\eta) ~Q^m_{n-1 / 2}(\cosh\eta^') </math>
	<math>~=</math>	<math>~ \frac{ [ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^')]^{1 /2 } }{\pi a \sqrt{\sinh\eta^'} \sqrt{\sinh\eta} } \sum\limits^\infty_{m=0} (-1)^m \epsilon_m \cos[m(\psi - \psi^')] </math>
		<math>~ \times \sum\limits^\infty_{n=0} \epsilon_n \cos[n(\theta - \theta^')] \biggl\{ ~ \sqrt{ \frac{\pi}{2} }~\Gamma(n-m+\tfrac{1}{2}) \sqrt{\sinh\eta}~P^m_{n-1 / 2}(\cosh\eta) \biggl\}\biggr\{ ~\sqrt{ \frac{2}{\pi} }~\frac{\sqrt{\sinh\eta^'}}{\Gamma(n + m + \tfrac{1}{2})} Q^m_{n-1 / 2}(\cosh\eta^') \biggr\} </math>

The term contained within the first set of curly braces on the right-hand side of this expression can be replaced by the derived expression labeled ①, above, and simultaneously the term contained within the second set of curly braces can be replaced by the derived expression labeled ②. After making these substitutions, we have,

<math>~\frac{1}{|~\vec{x} - {\vec{x}}^{~'} ~|} </math>	<math>~=</math>	<math>~ \frac{ [ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^')]^{1 /2 } }{\pi a \sqrt{\sinh\eta^'} \sqrt{\sinh\eta} } \sum\limits^\infty_{m=0} (-1)^m \epsilon_m \cos[m(\psi - \psi^')] </math>
		<math>~ \times \sum\limits^\infty_{n=0} \epsilon_n \cos[n(\theta - \theta^')] \biggl\{ ~ (-1)^{-n}Q^n_{m-1 / 2}(\coth\eta) \biggl\}\biggr\{ ~(-1)^{-m} P^{-n}_{m-1 / 2}(\coth\eta^') \biggr\} </math>
	<math>~=</math>	<math>~ \frac{ [ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^')]^{1 /2 } }{\pi a \sqrt{\sinh\eta^'} \sqrt{\sinh\eta} } \sum\limits^\infty_{m=0} \epsilon_m \cos[m(\psi - \psi^')] </math>
		<math>~ \times \sum\limits^\infty_{n=0} (-1)^{n} \epsilon_n \cos[n(\theta - \theta^')] Q^n_{m-1 / 2}(\coth\eta) P^{-n}_{m-1 / 2}(\coth\eta^') \, , </math>

where, in writing this last expression we have acknowledged that, since <math>~n</math> is either zero or a positive integer, <math>~(-1)^{-n} = (-1)^n</math>. Next we draw upon the "Key Equation" relation,

	<math>~ Q_\nu[t t^' - (t^2-1)^{1 / 2} (t^{'2} - 1)^{1 / 2} \cos\psi] </math>	<math>~=</math>	<math>~ Q_\nu(t) P_\nu(t^') + 2\sum_{n=1}^\infty (-1)^n Q^n_\nu(t) P^{-n}_\nu(t^') \cos(n\psi) </math>
A. Erdélyi (1953):  Volume I, §3.11, p. 169, eq. (4)

Valid for:

<math>~t, t^'</math>  real

<math>~1 < t^' < t</math>

<math>~\nu \ne -1, -2, -3, </math> …

<math>~\psi</math>   real

which, after making the substitutions, <math>~\nu \rightarrow (m - \tfrac{1}{2})</math> and <math>~\psi \rightarrow (\theta - \theta^')</math>, and incorporating the Neumann factor, <math>~\epsilon_n</math>, becomes,

<math>~ Q_{m - \frac{1}{2} }\{ t t^' - (t^2-1)^{1 / 2} (t^{'2} - 1)^{1 / 2} \cos[(n(\theta- \theta^')] \} </math>

<math>~=</math>

<math>~ \sum_{n=0}^\infty (-1)^n \epsilon_n Q^n_{m - \frac{1}{2} }(t) P^{-n}_{m - \frac{1}{2} }(t^') \cos[(n(\theta- \theta^')] \, . </math>

Finally, after making the associations, <math>~t \rightarrow \coth\eta</math> and <math>~t^' \rightarrow \coth\eta^'</math>, this last expression allows us to rewrite Wong's (1973) Green's function in a significantly more compact form, namely,

<math>~\frac{1}{|~\vec{x} - {\vec{x}}^{~'} ~|} </math>

<math>~=</math>

<math>~ \frac{ [ (\cosh\eta - \cos\theta)(\cosh \eta^' - \cos\theta^')]^{1 /2 } }{\pi a \sqrt{\sinh\eta^'} \sqrt{\sinh\eta} } \sum\limits^\infty_{m=0} \epsilon_m \cos[m(\psi - \psi^')] Q_{m - \frac{1}{2}}(\Chi) \, , </math>

where the argument, <math>~\Chi</math>, of the toroidal function, <math>~Q_{m - \frac{1}{2}}</math>, is,

<math>~\Chi</math>	<math>~\equiv</math>	<math>~ t t^' - (t^2-1)^{1 / 2} (t^{'2} - 1)^{1 / 2} \cos[(n(\theta- \theta^')] </math>
	<math>~=</math>	<math>~ \coth\eta \coth\eta^' - (\coth^2\eta-1)^{1 / 2} (\coth^2\eta'- 1)^{1 / 2} \cos[(n(\theta- \theta^')] </math>
	<math>~=</math>	<math>~ \frac{\cosh\eta \cosh\eta^'}{\sinh\eta \sinh\eta^'} - \biggl[ \frac{1}{\sinh^2\eta} \biggr]^{1 / 2} \biggl[ \frac{1}{\sinh^2\eta'}\biggr]^{1 / 2} \cos[(n(\theta- \theta^')] </math>
	<math>~=</math>	<math>~ \frac{\cosh\eta \cosh\eta^' - \cos[(n(\theta- \theta^')] }{\sinh\eta \sinh\eta^'} \, . </math>

As Presented in by Cohl & Tohline (1999)

This last, compact Green's function expression — which we have derived, here, from Wong's (1973) published Green's function by drawing strategically upon a variety of special function relations — precisely matches the "compact cylindrical Green's function expression" that has been derived independently by Cohl & Tohline (1999), namely,

<math>~ \frac{1}{|\vec{x} - \vec{x}^{~'}|}</math>	<math>~=</math>	<math>~ \frac{1}{\pi \sqrt{\varpi \varpi^'}} \sum_{m=0}^{\infty} \epsilon_m \cos[m(\psi - \psi^')] Q_{m- 1 / 2}(\Chi) </math>
	<math>~=</math>	<math>~ \frac{1}{a\pi} \biggl[ \frac{(\cosh\eta^' - \cos\theta^')}{\sinh\eta^' } \frac{(\cosh\eta - \cos\theta)}{\sinh\eta } \biggr]^{1 / 2} \sum_{m=0}^{\infty} \epsilon_m \cos[m(\psi - \psi^')] Q_{m- 1 / 2}(\Chi) \, . </math>

Note from J. E. Tohline (June, 2018):  To our knowledge, this is the first time that these two separately derived Green's function expressions have been shown to be identical. See, however, the earlier identification of new addition theorems in association with equations (49) and (50) of Cohl et al. (2000).

See Also

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