Stirling-number refinement of Lech's inequality for monomial ideals

Let (R,m)=k[[x0,,xd]](R,\mathfrak{m})=k[[x_0,\ldots,x_d]], and let IRI\subseteq R be an m\mathfrak{m}-primary monomial ideal. Let \stirlingIId+1k\stirlingII{d+1}{k} denote the Stirling numbers of the second kind. Then

e(I)k=0d(1)k(d+1k)!(dk)\stirlingIId+1d+1k1i1<<ikd(RI+(xi1,,xik))\operatorname{e}(I)\leq\sum_{k=0}^{d}(-1)^k\frac{(d+1-k)!}{\binom{d}{k}}\stirlingII{d+1}{d+1-k}\sum_{1\leq i_1<\cdots<i_k\leq d}\ell\left(\frac{R}{I+(x_{i_1},\ldots,x_{i_k})}\right)

and equivalently

e(I)=k=0d(1)dk(k+1)!(dk)\stirlingIId+1k+11i1<<idkd(RI+(xi1,,xidk)).\operatorname{e}(I)=\sum_{k=0}^{d}(-1)^{d-k}\frac{(k+1)!}{\binom{d}{k}}\stirlingII{d+1}{k+1}\sum_{1\leq i_1<\cdots<i_{d-k}\leq d}\ell\left(\frac{R}{I+(x_{i_1},\ldots,x_{i_{d-k}})}\right).

Stirling-number Lech conjecture. The first displayed inequality, together with the equivalent reindexing given in the source, holds for every m\mathfrak{m}-primary monomial ideal II. It is known in dimension three, where d=2d=2, but remains conjectural in general.

Sources & referencesView supporting material

Primary source

Linquan Ma and Ilya Smirnov, “Lech-Mumford constant and stability of local rings”, arXiv:2508.19893 (2025).

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