Simple symmetric Venn diagrams with 17 and 19 curves

Abstract

We exhibit simple, rotationally symmetric Venn diagrams with 17 curves and with 19 curves: n Jordan curves carried to one another by rotation through 2π/n, with every one of the 2^n regions present and connected and, since the diagrams are simple, every crossing on exactly two curves. Symmetric Venn diagrams exist for every prime number of curves (Griggs, Killian and Savage, 2004), but those diagrams have many curves through a point; simple ones were known only up to 13 curves (Mamakani and Ruskey, 2014). Four 17-curve and nine 19-curve diagrams were found by a Metropolis walk on rotation-invariant quadrangulations of the sphere in which regions may temporarily be duplicated, started from the Griggs-Killian-Savage diagram with its multiple crossings resolved. Every diagram is given by a machine-checkable certificate; one certificate of each size has been verified by a formal proof in Lean 4. All of the diagrams are non-monotone, which is why the crossing-sequence searches that found the 11- and 13-curve diagrams could not have found them.