Talk:Sturm-Liouville theory

Numbered equations

Hi Daniel,

I had to transform the templates provided in the WP article for numbered equations into plain old wikimarkup. I attempted to bring these templates over from WP, but they called many other templates and when I got them all over, the combined result didn't work. I decided to use a simple bit of html that defined a span with right justification and also defined an anchor. To reference the equation you then only need to insert a mediawiki markup referencing the anchor. It wouldn't be hard to turn this all into two templates if you think that would be useful. Dan Nessett 19:28, 2 September 2009 (UTC)

Redacted comment. I see it now.

Error

It seems to me that the article contains an error. Consider the equation:

$Lu={1 \over w(x)}\left(-{d \over dx}\left[p(x){du \over dx}\right]+q(x)u\right)$

Contrary to what is implied in the article, the operator L is not self-adjoint, unless 1/w(x) commutes with the operator to its right. This in general not the case. The proper way to transform is

$Lu=wu\;\Longrightarrow \;w^{-1/2}Lw^{-1/2}w^{1/2}u=w^{1/2}u\;\Longrightarrow \;{\tilde {L}}\,{\tilde {u}}={\tilde {u}}$

with

${\tilde {L}}\equiv w^{-1/2}Lw^{-1/2}\quad {\hbox{and}}\quad {\tilde {u}}\equiv w^{1/2}u$

Since w(x) is positive-definite w(x)^−½ is well-defined and real. The operator ${\tilde {L}}$ is self-adjoint.

--Paul Wormer 08:09, 14 October 2009 (UTC)