## What theorem of Liouville’s is Gian-Carlo Rota referring to here?

I am very curious about this remark in Lesson Four of Rota's talk, Ten Lessons I Wish I Had Learned Before I Started Teaching Differential Equations:

"For second order linear differential equations, formulas for changes of dependent and independent variables are known, but such formulas are not to be found in any book written in this century, even though they are of the utmost usefulness.

"Liouville discovered a differential polynomial in the coefficients of a second order linear differential equation which he called the invariant. He proved that two linear second order differential equations can be transformed into each other by changes of variables if and only if they have the same invariant. This theorem is not to be found in any text. It was stated as an exercise in the first edition of my book, but my coauthor insisted that it be omitted from later editions."

Does anyone know where to find this theorem?

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Hyperbole is the worst thing in the universe, and Gian-Carlo Rota used it frequently. In this article, he wrote: "The Administrative Director of the MIT mathematics department, who exercises supreme authority upon the faculty's teaching, has only to wave a copy of my book at me, while staring at me in silence. At her prompting, I bow and fall into line; I will be the lecturer in the dreaded course for one more year, and[...]". Some years ago I called this sentence to the attention of the said "administrative director, and she said there's not a word of truth in it. – Michael Hardy Apr 22 2011 at 21:46
Hyperbole is far from the worst thing in the universe. Rota's flair for the dramatic is part of what made him such an engaging lecturer. – AVS Apr 23 2011 at 2:03
Um.....AVS, did you miss something? But then, who needs rhetorical questions? – Michael Hardy Apr 23 2011 at 5:58
BTW, I plagiarized that line from a facebook friend. He wrote it on his wall and put the period after the word "universe" and didn't go on from there. Maybe adding more stuff confuses the issue. – Michael Hardy Apr 23 2011 at 6:04
I apologize for misinterpreting your comment. Your meaning and tone would have been entirely clear had you ended your comment at the word universe as you suggest (and I'm sure Rota would have heartily approved). – AVS Apr 23 2011 at 9:43

See E. Hille, Ordinary differential equations in the complex domain, Wiley, New York, 1976. The Liouville transformation is given on Page 179. The invariant mentioned by Rota is the function $Q(z)$ appearing as a coefficient of the equation in the canonical form.

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 Thank you very much, I will look up this reference. – lkeer Apr 23 2011 at 7:21

Kamke's classic compendium [1] of ODE solutions and solution methods displays this invariant in Part I, equation §25.1(4). The invariant is given for the more famous equations (Bessel, Legendre, hypergeometric, ...) of the large list of second order linear equations in Part III Chapter II.

[1] Kamke, E. Differentialgleichungen: Lösungen und Lösungernethoden. Vol. 1. First published in 1944.

Unfortunately, it seems that this book never appeared in English translation.

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