Determine whether combinatorics distinguish free and nearly free arrangements in the exceptional range

Determine whether there exist a free line arrangement A and a nearly free line arrangement B with the same combinatorics, specifically in the case d1 = m + 1 and 2m + 3 ≤ d, where m denotes the maximal multiplicity and d the number of lines.

Background

The paper discusses the extent to which the freeness of a line arrangement can be decided from its combinatorics. It identifies d1 = m + 1 with 2m + 3 ≤ d as a regime in which freeness is not readily determined by the available numerical criteria.

The authors exhibit two arrangements with identical global Tjurina number: a free arrangement with exponents (m + 1, m + 1) and a nearly free arrangement with exponents (m, m + 3, m + 3). However, they do not establish whether such arrangements can have the same combinatorics, leaving the existence question unresolved.

References

For instance, the free arrangement A with exponents (m + 1, m + 1) and the nearly free arrangement B with exponents (m, m + 3, m + 3) satisfy τ (A) = τ (B). We do not know whether such a pair of arrangements A, B exists, such that A and B have the same combinatorics.

On the module of derivations of a line arrangement  (2503.01624 - Dimca, 3 Mar 2025) in Remark 5.4, Section 5, p. 20