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Oscillations of PP Tori in the Slim Torus Limit
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Statement of the Eigenvalue Problem
Here, we build on our discussion in an accompanying chapter in which five published analyses of nonaxisymmetric instabilities in PapaloizouPringle tori were reviewed: The discovery paper, PP84, and papers by four separate groups that were published within a couple of years of the discovery paper — Papaloizou & Pringle (1985), Blaes (1985), Kojima (1986), and Goldreich, Goodman & Narayan (1986). Following the lead of Blaes (1985; hereafter Blaes85), in particular, we have shown that the relevant eigenvalue problem is defined by the following 2^{nd}order PDE,



where, is the dimensionless enthalpy perturbation. Making the substitution,
this governing equation — now, a onedimensional, 2^{nd}order ODE — becomes,



Making the additional substitution,
and appreciating that,






the governing ODE becomes,












Finally, then, making the independent variable substitution,
in which case,






and,















This matches equation (3.9) of Blaes85. According to Blaes (1985), this equation "… is a standard eigenvalue problem whose only solutions are the Jacobi polynomials …"
Solution
My own background training and experience has not previously exposed me to the general class of Jacobi polynomials. In my effort to understand this class of polynomials and, specifically, their relationship to the Singular SturmLiouville Equation, I have found the following references to be useful:
 PDFformatted class notes presented by Professor Haijun Yu
 PDFformatted class notes presented by Professor Russell Herman
 A couple of scanned pages from §22 of Abramowitz & Stegun (1972) posted online by Colin Macdonald
Singular SturmLiouville Problem
Drawing on Theorem 3.16 from Yu's class notes, we find that each one of the set of Jacobi polynomials, , is an eigenfunction of the singular SturmLiouville problem whose mathematical definition is provided by the 2^{nd}order ODE,



where the differential operator,






and the corresponding j^{th} eigenvalue is,



(Note that we have used "j" instead of the more traditional use of "n" to identify the specific Jacobi polynomial, because we are already using "n" to denote the polytropic index.) According to Theorem 3.17 from Yu's class notes, for each specified value of the index, , the eigenfunction solution to this eigenvalue problem — that is, the relevant Jacobi polynomial — is,



As an example, let's carry out the prescribed differentiation to determine the Jacobi polynomial for the case of .



























Table 1 presents this "j=2" eigenfunction — along with its corresponding eigenfrequency — as well as the eigenfunctions and eigenfrequencies for "j=0" and "j=1."
Table 1: Example Jacobi Polynomials  










See also, eqs. (35)(37) of Wolfram MathWorld. 
Okay, so now I understand how to identify the complete set of eigenvectors for a stability problem in which the governing ODE takes the form of the singular SturmLiouville equation. But, with the recognition that the Blaes85 ODE does not have precisely the same form as the singular SturmLiouville equation, it is unclear how we will win by pursuing this specific line of investigation.
Jacobi Differential Equation
Instead, let's pursue the following lead. Wolfram MathWorld provides a discussion of the Jacobi Differential Equation. Equation (1) of this reference appears to be identical to the singular SturmLiouville equation presented above. However, in addition, this MathWorld chapter points out that Zwillinger (1997, p. 123) gives a related differential equation that is referred to as "Jacobi's equation," namely,



If we rewrite our abovederived ODE — which is identical to eq. (3.9) of Blaes85 — as,



we see that its form is identical to this socalled "Jacobi's equation." The overlap is complete if we make the following parameter associations:









We can infer from this third association that, for the j^{th} eigenfunction solution of the governing eigenvalue problem, the relevant oscillation frequency is,









This is identical to the eigenfrequency identified in eq. (3.12) of Blaes85. So, we must be on the right track! Now I just need to figure out how to express the corresponding eigenfunction in terms of Jacobi polynomials.
From expression 22.5.1 on p. 777 of A&S, we understand that,



In addition, we see from eq. (3.11) of Blaes85 that, to within a factor of a leading constant, the physically relevant eigenfunction is,



By association, we conclude from this that,









From this third association, we conclude that the argument of the Jacobi polynomial should be,
Following up on this clue, let's plug this coordinate transformation into Blaes85 expression (3.9) and see whether it takes the form of the SturmLiouville ODE. Specifically, making the substitutions,
and
we have,












which is precisely in the standard SturmLiouville format. The j^{th} eigenfunction should therefore be, , and the associated eigenfrequency should be,



where,






Adding these two expressions together implies that,



while subtracting them implies,



Hence, the eigenfrequency is,












Again, this precisely matches equation (3.12) from Blaes85, so we appear to be on the right track. Putting this all together, Table 2 presents the first three eigenvectors that are relevant to the stability analysis of slim, PP tori.
Material that appears after this point in our presentation is under development and therefore
may contain incorrect mathematical equations and/or physical misinterpretations.
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Intermediate Detail  

where, from Table 1,
Hence,

Table 2: Example Slim PPTori Eigenvectors  










Summary
The spatial structure of the normalized enthalpy perturbation associated with each mode of oscillation in a slim PP torus is,



and the associated oscillation frequency is,



Because the oscillation frequency is always positive, all modes are dynamically stable. Because the meridional plane amplitude is everywhere real, the azimuthal distortion associated with each nonaxisymmetric mode, , is completely uninteresting.
See Also
© 2014  2020 by Joel E. Tohline 