The interval conjecture for region counts of circles with pairwise intersections

Let CnC_n be the set of numbers of regions formed by nn circles in the plane, in an arrangement not in general position, such that every two circles have at least one common point. Let LnL_n be the set of numbers of regions formed by nn distinct lines in the plane in an arrangement not in general position. The maximal elements of LnL_n and CnC_n are respectively n(n1)2+1\frac{n(n-1)}{2}+1 and n(n1)+2n(n-1)+2. Interval conjecture. The set CnC_n contains every integer in the interval from n(n1)2+1\frac{n(n-1)}{2}+1 through n(n1)+2n(n-1)+2. This would determine a complete consecutive range of attainable region counts for the circle arrangements under consideration, extending the relation between the possible counts for circles and lines; the supplied text gives no resolution.

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Primary source

I. Shnurnikov, “What is the number of decompositions of torus into given number of regions by unions of geodesics?”, arXiv:1501.00893 (2014).

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