Conjecture on the components of the rank-two locus of the cubic discriminant

Let VkV_k be the locus of cubic surfaces for which the matrix MM has rank at most 20k20-k, and let N2N_2 denote the locus of binodal cubic surfaces. The 19×1919\times 19 minors of MM define V2V_2. Conjecture on V2V_2. The locus V2V_2 has at least two irreducible components parameterising binodal and cuspidal cubic surfaces. In particular,

N2V2N_2\subset V_2

and the 19×1919\times 19 minors of MM vanish on N2N_2. This is suggested by the computation that a general plane section of V2V_2 consists of 400400 points, matching the 280280 binodal and 120120 cuspidal cubic surfaces in the singular locus of the discriminant curve; the claim remains conjectural, although the binodal inclusion is verified in a normal-form computation.

Sources & referencesView supporting material

Primary source

Dominic Bunnett and Hanieh Keneshlou, “Determinantal representations of the cubic discriminant”, arXiv:1909.05579 (2019).

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