General minor closure of Mandel oriented matroids

Establish whether the class of Mandel oriented matroids is minor-closed without restricting deletions to those that preserve rank or imposing uniformity or low-rank hypotheses.

Background

The paper establishes that Mandel oriented matroids remain Mandel after contractions and after deletions that do not reduce rank. It also proves minor closure for rank at most four and for uniform Mandel oriented matroids. The unrestricted minor-closure question therefore concerns the cases not covered by these results.