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arXiv · 1806.05295

A homological characterization for freeness of multi-arrangements

Michael DiPasquale
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Abstract

Building on work of Brandt and Terao in their study of $k$-formality, we introduce a co-chain complex associated to a multi-arrangement and prove that its cohomologies determine freeness of the associated module of multi-derivations. This provides a new homological method for determining freeness of arrangements and multi-arrangements. We work out many applications of this homological method. For instance, we prove that if a multi-arrangement is free then the underlying arrangement is $k$-formal for all $k\ge 2$. We also use this method to completely characterize freeness of certain families of multi-arrangements in moduli, showcasing how the geometry of multi-arrangements with the same intersection lattice may have considerable impact on freeness. New counter-examples to Orlik's conjecture also arise in connection to this latter analysis.

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Michael DiPasquale. 2018-06-13. A homological characterization for freeness of multi-arrangements. https://arxiv.org/abs/1806.05295

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