The cluster-count criterion for secant and initial ideals

Let T\mathcal{T} be a binary phylogenetic [n][n]-tree. For each kk, let ckc_k denote the number of kk-clusters in T\mathcal{T}, and let JTJ_{\mathcal{T}}, I2,nI_{2,n}, ω\omega, and τ\tau be as above, with

JT=τinω(I2,n).J_{\mathcal{T}}=\tau\cdot \operatorname{in}_{\omega}(I_{2,n}).

The cluster-count criterion. One has

τinω(I2,n{r})=JT{r}\tau\cdot \operatorname{in}_{\omega}\left(I_{2,n}^{\{r\}}\right)=J_{\mathcal{T}}^{\{r\}}

if and only if

k=2r(rk+1)ck<2r22r.\sum_{k=2}^{r}(r-k+1)c_k<2r^2-2r.

This proposed modification is motivated by dimension bounds and examples showing failure of equality when the cluster-count expression is too large. The supplied text does not state whether the criterion has been proved or disproved.

Sources & referencesView supporting material

Primary source

Colby Long, “Initial Ideals of Pfaffian Ideals”, arXiv:1610.06524 (2016).

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