Asymptotic Lech Bounds and Bertini Flags

Abstract

For a $d$-dimensional Noetherian local ring $(R,\mathfrak{m})$, with $d>0$, let $c_{\infty }(R)$ be the limit, as $N\to \infty$, of the suprema of $e_R(I)/(d!{\ell }_R(R/I))$ over ideals $I$ with $\sqrt{I}=\mathfrak{m}$ and ${\ell }_R(R/I)>N$. We prove that if $R$ is complete, reduced, equidimensional, and Cohen--Macaulay on the punctured spectrum, with $d\ge 2$ and infinite residue field, then every nonzerodivisor $x\in \mathfrak{m}$ satisfies \[ c_{\infty}(R)\leq \max\left\{c_{\infty}(R/xR), \sup_{\mathfrak{p}\ne\mathfrak{m}}e(R_{\mathfrak{p}})\right\}. \] As an application, we prove the asymptotic Lech conjecture of Huneke, Ma, Quy, and Smirnov in arbitrary characteristic: $c_{\infty }(R)=1$ whenever $\hat{R}$ has isolated singularities. This also settles the conjecture of Ma and Smirnov that the Lech--Mumford constant $c_{\mathrm{LM}}(R)$ is attained when $\hat{R}$ has isolated singularities and $c_{\mathrm{LM}}(R)>1$. For reduced equidimensional Cohen--Macaulay standard graded algebras of dimension $\ge 2$ over infinite perfect fields, we prove attainment and rationality of $c_{\mathrm{LM}}$ for the completed vertex local rings of sufficiently high Veronese subrings. In characteristic zero, a refinement of our method produces nonisolated klt affine cones in every dimension $d\ge 3$, with $d-2$-dimensional singular loci, whose completed vertex local rings have $c_{\infty }=1$ and arbitrarily large $c_{\mathrm{LM}}$.

arXiv comment32 pages