TITLE: Kneser theorem about the Klein bottle
QUESTION [7 upvotes]: I know that in $1923$ H. Kneser showed that a continuous flow in a Klein bottle without singular points has a periodic trajectory. The original article is this, but does anyone know another old or new proof of this result? (So far I have found the article by Kneser, the article by Nelson G. Markley and some works that use the results of these articles, I am looking for some other idea focused on proving the result mentioned at the beginning) I would really like to read this result, I tried to do it from your original article but the language is too complicated for me. I searched on the internet but found almost nothing about the proof. I asked here but I didn't find any answer even with bounty. I hope to have some help it would help me a lot.

REPLY [4 votes]: It sounds like you are looking for a textbook.  I have read bits of Introduction to the geometry of foliations: Part A by Hector and Hirsch.  It is well-written, with pictures! They give Kneser's theorem on pages 62-65, after developing the necessary theory.