The small MCM conjecture for tame bi-partite toric rings

Let SS be a regular local ring of characteristic p>0p>0, let ΓNn\Gamma\subset\mathbb N^n be a full semigroup, and let φ ⁣:ΓNd\varphi\colon\Gamma\to\mathbb N^d and χ ⁣:ΓS\chi\colon\Gamma\to S^* be respectively a homomorphism and an SS-character. Let R:=R(S,Γ,φ,χ)R:=\mathcal R(S,\Gamma,\varphi,\chi) be the bi-partite toric ring defined by this data. Write satS(1;FqR)\operatorname{sat}_{S}(*1;\mathbf{F}_{q*}R) for the indicated SS-saturation in the Frobenius transform, and suppose, in the additional situation described in the source, that RR has a unique dd-dimensional prime p\mathfrak p and that φ\varphi is tame.

Small MCM conjecture for tame bi-partite toric rings. For some sufficiently high power qq of pp, RR admits the small MCM

satS(1;FqR).\operatorname{sat}_{S}(*1;\mathbf{F}_{q*}R).

Moreover, under the additional hypotheses, p\mathfrak p annihilates this module and

satS(1;FqR)F ⁣int(R/p)=(R/p)pow.\operatorname{sat}_{S}(*1;\mathbf{F}_{q*}R)\cong \mathbf{F}_{\!}^{\operatorname{int}}(R/\mathfrak p)=(R/\mathfrak p)^{\operatorname{pow}}.

The statement extends the proposed small MCM construction beyond the principal families considered earlier. The source notes that the tameness assumption is necessary in an example; the conjecture itself remains open.

Sources & referencesView supporting material

Primary source

Hans Schoutens, “Maximal Cohen-Macaulay modules over local toric rings”, arXiv:1408.6220 (2014).

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