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? 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.

  • $\begingroup$ Nice question: if no one answers it here, I advice you to post it also on the MathOverflow, our sister site. $\endgroup$ May 16, 2022 at 11:32
  • $\begingroup$ @DanieleTampieri Thank you very much, I'll wait for the time to end and if I don't get an answer I'll post it on the other side... $\endgroup$
    – Zaragosa
    May 16, 2022 at 21:09
  • $\begingroup$ also at mathoverflow.net/q/423108/11260 $\endgroup$ May 27, 2022 at 5:51


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