The Milnor-number equality criterion including exceptional vertices

Let fK[[x,y]]f\in\mathbb{K}[[x,y]] be a reduced power series, and let T\mathcal{T} be a minimal tree of ff. Let V\mathcal{V} be the set of vertices of T\mathcal{T}, let A0\mathcal{A}_0 be the distinguished set of additional vertices in the tree construction, and let NvN_v be the integer attached to each vVA0v\in\mathcal{V}\cup\mathcal{A}_0. Milnor-number equality criterion. The equality between the Milnor number and the characteristic-independent invariant holds if and only if CharK\operatorname{Char}\mathbb{K} does not divide NvN_v for all vVA0v\in\mathcal{V}\cup\mathcal{A}_0. The surrounding discussion identifies this as the equality case of the characteristic-zero formula and the corresponding positive-characteristic inequality, but the supplied text gives no resolution status.

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Primary source

Enrique Artal Bartolo and Pierrette Cassou-Noguès, “Milnor number of plane curve singularities in arbitrary characteristic”, arXiv:2409.13520 (2025).

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