Third-Order Weierstrass Points on Plane Quartics

Source abstract

For a smooth plane quartic $C$, the $3$-Weierstrass points are the $300$ points (counted with weights) at which some plane cubic has contact order at least ten with $C$. Revisiting the work of Kamel and Elshareef, we write the third Veronese Wronskian as an $\text{SL}_3$-covariant form $Ω_3$ of degree $75$, given by a determinant of iterated Hamiltonian derivatives, and explain the special $3$-Weierstrass points through contact cubics and homologies preserving $C$. We determine $Ω_3$ for the Fermat, Komiya--Kuribayashi, Klein and Picard quartics; for the Klein quartic $Ω_3\equivΦ_6^2\,Φ_{21}\,(3Φ_{14}^3+21962\,Φ_6^7)$ modulo the curve, up to a non-zero scalar. Finally, the dectactic points of the Fermat quartic are not $4$-Weierstrass points, which disproves a conjecture of Kamel and Elshareef. All computations are supported by \texttt{SINGULAR} scripts.