The simpliciality conjecture for arrangements with fewer than half double points

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Let A\mathcal{A} be an arrangement consisting of nn pseudolines, and let t2At^\mathcal{A}_2 denote its number of double points. Its characteristic polynomial χ(A,t)\chi(\mathcal{A},t) need not split over R\mathbb{R}. Simpliciality conjecture. If

t2A<n2,t^\mathcal{A}_2<\frac{n}{2},

then A\mathcal{A} is simplicial. The conjecture is motivated by the fact that the only two known arrangements with fewer than n/2n/2 double points are simplicial, while equality can also occur for non-simplicial arrangements.

References

Primary source

David Geis, “Combinatorics of free and simplicial line arrangements”, arXiv:1809.09362 (2019).

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