This is the Grünbaum–Rigby configuration (, 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:

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

Sign in to participate in the conversation

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

Use \( and \) for inline LaTeX, and \[ and \] for display mode.