Diagonal Specht ideals and their varieties

Source abstract

We study the diagonal Specht ideals $I_λ\subseteq\mathcal{R}_{m,n}:=\mathbb{C}[\mathbf{x}_1,\dots,\mathbf{x}_m]$, $\mathbf{x}_r=(x_{r,1},\dots,x_{r,n})$, generated by the $λ$-isotypic component of $\mathcal{R}_{m,n}$ for the diagonal action of $S_n$. We characterize the set of zeros of $I_λ$ as $V_λ=\bigcup_{μ\not\trianglelefteqλ}H_μ$ and prove that $λ\mapsto V_λ$ is an isomorphism of posets from $(\mathcal{P}_n,\trianglelefteq)$ to $(\{V_λ:λ\in \mathcal{P}_n\},\supseteq)$ for all $m$, while $λ\mapsto I_λ$ is an isomorphism of posets from $(\mathcal{P}_n,\trianglelefteq)$ to $(\{I_λ:λ\in \mathcal{P}_n\},\subseteq)$ if and only if $m=1$ or $n\leq3$. Unlike the case $m=1$, where Specht ideals are always radical, radicality in the diagonal setting depends on $λ$ and $m$. We prove radicality for hook partitions of length at most three by providing explicit Gröbner bases, and we develop three criteria for non-radicality via content, multidegree, and total degree, showing in particular that $I_λ$ is not radical for any non-hook partition $λ$ when $m\geq\operatorname{len}(λ)$. Using the $\operatorname{GL}_m(\mathbb{C})$-action on $\mathcal{R}_{m,n}$, we show that $I_λ$ is radical for all $m$ if and only if it is radical for $m=n$. Finally, for $m\geq2$, we prove that $\mathcal{R}_{m,n}/I_λ$ is Cohen-Macaulay if and only if $λ=(n)$ or $λ=(n-1,1)$, and the same is true for $\mathcal{R}_{m,n}/\operatorname{rad}(I_λ)$.