Further Questions about Tetrads
13. Can four quadrilaterals form a tetrad?

No: That's impossible. Does anyone have an idea for a short proof?

Here an interesting attempt is shown. The vertices of the inner tile (yellow) touch all other tiles in an edge. No common vertices occur.

This attempt leads to a new question: Are there tetrads where no pair of the four regions has common vertices?
See question 16.

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 Walter Trump, Nürnberg, Germany, ms(at)trump.de, © 2020-02-12 (last modified: 2020-02-12)