The converse conjecture for F-pure thresholds and Segre-product tangent cones

Let (R,m)(R,\mathfrak{m}) be a dd-dimensional F-finite F-pure non-Q\mathbb{Q}-Gorenstein normal local ring of characteristic p>0p>0. Let c(m)\mathrm{c}(\mathfrak{m}) denote its F-pure threshold, and let grm(R)\mathrm{gr}_{\mathfrak{m}}(R) be the associated graded ring of RR with respect to m\mathfrak{m}. Converse conjecture. If

c(m)=d1,\mathrm{c}(\mathfrak{m})=d-1,

then

grm(R)k[Xij1id1, j=1,2]/(Xk1Xl2Xk2Xl11kld1).\mathrm{gr}_{\mathfrak{m}}(R)\cong k[X_{ij}\mid 1\le i\le d-1,\ j=1,2]/(X_{k1}X_{l2}-X_{k2}X_{l1}\mid 1\le k\le l\le d-1).

This is the expected converse of the preceding example, which shows that the stated value of the F-pure threshold occurs for the completion of a Segre product that is not Q\mathbb{Q}-Gorenstein. The source provides no resolution of this expectation.

Sources & referencesView supporting material

Primary source

Shunsuke Takagi and Kei-ichi Watanabe, “On F-pure thresholds”, arXiv:math/0312486 (2004).

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