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arxiv: 1903.11925 · v1 · pith:XRTWUDZSnew · submitted 2019-03-28 · 🧮 math.CO

Combinatorially formal arrangements are not determined by their points and lines

classification 🧮 math.CO
keywords combinatorialformalityconditionformalhyperplanesliftlinesmatroid
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An arrangement of hyperplanes is called formal, if the relations between the hyperplanes are generated by relations in codimension 2. Formality is not a combinatorial property, raising the question for a characterization for combinatorial formality. A sufficient condition for this is if the underlying matroid has no proper lift with the same points and lines. We present an example of a matroid with such a lift but no non-formal realization, thus showing that above condition is not necessary for combinatorial formality.

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