Talk:Sturm-Liouville theory
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A bit wordy.
This article is good and easy to read, but too wordy. The distraction about the linear polynomials being eigenfunctions of the Laplacian is unnecessary. All one needs to do is write the formula for the eigenvectors (exp(ikx) where k is even), maybe something like "one checks that exp(ikx), k even, solves the ode."
Similarly, it should be made more clear exactly why it is that arbitrary functions may be written as superpositions of such basic waves. The short answer (applicable to the examples in the article) is Fourier series. The long answer is that the BVP Lu=f subject to Dirichlet boundary data on a nice bounded domain when L is (say) coercive is a map F:f-->u from L^2 to H^1 that is continuous; hence the composition F with the inclusion map i:H^1-->L^2 (which is a compact linear map) gives a compact linear map G=iF, G:L^2-->L^2. This map is self-adjoint if L is self-adjoint, and so it admits a basis of eigenvectors. The eigenvalues of G must accumulate at the origin and nowhere else, hence the eigenvalues of L accumulate at ∞ and nowhere else. This also proves that the set of basic wave functions will necessarely span L^2, even if they are not exp harmonics.
One should then refer to the Sobolev space article.
I may make the changes some other day if nobody else does it.
Loisel 09:46, 6 Jun 2004 (UTC)
I've rewritten the first half; the second half will come later. Loisel 19:02, 7 Jul 2004 (UTC)
The "normal modes" section is a bit strange. The equation isn't in the Lu=∑D^p(a_{pq}D^qu)+bu=0 form, nor is it in the eigenvalue Lu=λu form. Furthermore, the problem solved in that section is not that of eigenanalysis (for the purpose of solving Lu=f subject to homogeneous boundary conditions) but rather to solve Lu=0 subject to nonhomogeneous boundary conditions.
I think it's a good idea to have a multiple variable example of a S-L problem, and the wave equation is fine with me, but I think the wave operator (what do physicists call it?) and the S-L connection should be made explicit.
Loisel 06:51, 8 Jul 2004 (UTC)