Grünbaum's conjecture on unstretchable simplicial arrangements

A simplicial arrangement is a finite set of hyperplanes whose complement has only simplicial cones as connected components; in the projective-plane setting, these are arrangements of pseudolines. Two arrangements are isomorphic when they are equivalent under the relevant combinatorial notion, and an arrangement is unstretchable if it is not realizable by straight lines.

Grünbaum's conjecture. Up to isomorphism, there are only $5$ simplicial unstretchable arrangements of $15$ or $16$ pseudolines.

The paper reports further examples disproving this conjecture, so the claim is refuted. The broader classification of simplicial arrangements remains an open problem.