Invariants of plane curve singularities and Pl\"ucker formulas in positive characteristic

Abstract

We study classical invariants for plane curve singularities $f\in K[[x,y]]$, $K$ an algebraically closed field of characteristic $p\geq 0$: Milnor number, delta invariant, kappa invariant and multiplicity. It is known, in characteristic zero, that $\mu(f)=2\delta(f)-r(f)+1$ and that $\kappa(f)=2\delta(f)-r(f)+\mathrm{mt}(f)$. For arbitrary characteristic, Deligne prove that there is always the inequality $\mu(f)\geq 2\delta(f)-r(f)+1$ by showing that $\mu(f)-\left( 2\delta(f)-r(f)+1\right)$ measures the wild vanishing cycles. By introducing new invariants $\gamma,\tilde{\gamma}$, we prove in this note that $\kappa(f)\geq \gamma(f)+\mathrm{mt}(f)-1\geq 2\delta(f)-r(f)+\mathrm{mt}(f)$ with equalities if and only if the characteristic $p$ does not divide the multiplicity of any branch of $f$. As an application we show that if $p$ is "big" for $f$ (in fact $p > \kappa(f)$), then $f$ has no wild vanishing cycle. Moreover we obtain some Pl\"ucker formulas for projective plane curves in positive characteristic.Comment: 15 pages; final version; to appear in the Annales de l'Institut Fourie