The nonfree line-arrangement double-point conjecture

Let A{\mathcal A} be a projective line arrangement in PK2{\mathbf P}^2_{\mathbb K}, where K{\mathbb K} is a field of characteristic zero. Write n2(A)n_2({\mathcal A}) for the number of double points of A{\mathcal A}, and call A{\mathcal A} free when its associated arrangement is free.

Nonfree line-arrangement double-point conjecture. If A{\mathcal A} is not free, then

n2(A)>0.n_2({\mathcal A})>0.

The conjecture asks whether every nonfree arrangement over a characteristic-zero field has a double point. The supplied text gives experimental motivation but no proof or resolution.

Sources & referencesView supporting material

Primary source

Takuro Abe, “Double points of free projective line arrangements”, arXiv:1911.10754 (2019).

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