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Where can i find a correct proof of the following theorem of H.Hopf:

For every continuous curve $L$ in the plane with endpoints $A$ and $B$ such that distance $\vert AB \vert = 1$, and for any natural number $n$, there exists a chord (meaning a straight segment with endpoints on $L$) that is parallel to $AB$ and whose length equals $1/n$.

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    I can't find the proof, but it is in "H. Hopf, Uber die Sehen ebener Kontinuen und die Schleifen geschlossener Wege, Commentarii Mathematici Helvetici IX (1936-37), 303-319". If I remember correctly. – 2017-02-15
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    Very similar to [this question](http://math.stackexchange.com/questions/1276889/proving-that-there-exists-a-horizontal-chord-with-length-1-n-for-a-continuous?rq=1). – 2017-02-15

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