This is the Grünbaum–Rigby configuration (en.wikipedia.org/wiki/Gr%C3%BC), three overlaid regular heptagrams with 21 points and lines, 4 points per line, and 4 lines per point. Klein studied it in the complex projective plane in 1879, but it wasn't known to have a nice real realization until Grünbaum and Rigby (1990). Wikipedia editor "Tomo" (I'll let you figure out who that is) started a new article a month ago, and now it's on the front page of Wikipedia in the "Did you know" section.

@11011110 I am surprised that a complete list of these combinatorial configurations is not known, this seems to be well within the reach.

@dimpase It's only known up to 18 points and lines: oeis.org/A023994

Estimating from the growth rate, there could easily be half a trillion of these things for n=21.

A Mastodon instance for maths people. The kind of people who make $\pi z^2 \times a$ jokes.

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